diff --git a/src/obicompactvec/src/bitmatrix.rs b/src/obicompactvec/src/bitmatrix.rs
index c054ae0..72f8b05 100644
--- a/src/obicompactvec/src/bitmatrix.rs
+++ b/src/obicompactvec/src/bitmatrix.rs
@@ -299,9 +299,7 @@ impl PersistentBitMatrix {
             Self::Columnar(m) => PersistentBitVecBuilder::build_from(m.col(c), path),
             Self::Packed(m)   => m.col_persist(c, path),
             Self::Implicit { n_rows, .. } => {
-                let mut b = PersistentBitVecBuilder::new(*n_rows, path)?;
-                b.not();
-                Ok(b)
+                PersistentBitVecBuilder::new_ones(*n_rows, path)
             }
         }
     }
@@ -402,6 +400,12 @@ impl PersistentBitMatrixBuilder {
         PersistentBitVecBuilder::new(self.n, &path)
     }
 
+    pub fn add_col_ones(&mut self) -> io::Result<PersistentBitVecBuilder> {
+        let path = col_path(&self.dir, self.n_cols);
+        self.n_cols += 1;
+        PersistentBitVecBuilder::new_ones(self.n, &path)
+    }
+
     pub fn add_col_from(&mut self, src: &TempBitVec) -> io::Result<()> {
         src.make_persistent(&col_path(&self.dir, self.n_cols))?;
         self.n_cols += 1;
diff --git a/src/obicompactvec/src/bitvec.rs b/src/obicompactvec/src/bitvec.rs
index 966d57f..145bd63 100644
--- a/src/obicompactvec/src/bitvec.rs
+++ b/src/obicompactvec/src/bitvec.rs
@@ -145,6 +145,33 @@ impl PersistentBitVecBuilder {
         Ok(Self { mmap, n, path: path.to_path_buf() })
     }
 
+    /// Create an all-ones bit vector of length `n` at `path`.
+    ///
+    /// More efficient than `new(n, path)` + `not()`: the data is written as
+    /// 0xFF bytes in a single sequential pass, with no intermediate all-zeros state.
+    pub fn new_ones(n: usize, path: &Path) -> io::Result<Self> {
+        let nw        = n_words(n);
+        let file_size = HEADER_SIZE + nw * 8;
+        let mut file  = OpenOptions::new()
+            .read(true).write(true).create(true).truncate(true)
+            .open(path)?;
+        file.write_all(&MAGIC)?;
+        file.write_all(&[0u8; 4])?;
+        file.write_all(&(n as u64).to_le_bytes())?;
+        file.write_all(&vec![0xFFu8; nw * 8])?;
+        file.seek(SeekFrom::Start(0))?;
+        file.set_len(file_size as u64)?;
+        let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+        // Clear padding bits in the last word so trailing bits are always 0.
+        let rem = n % 64;
+        if rem != 0 {
+            let ptr   = mmap[HEADER_SIZE..].as_mut_ptr() as *mut u64;
+            let words = unsafe { std::slice::from_raw_parts_mut(ptr, nw) };
+            words[nw - 1] &= (1u64 << rem) - 1;
+        }
+        Ok(Self { mmap, n, path: path.to_path_buf() })
+    }
+
     pub fn build_from(source: &PersistentBitVec, path: &Path) -> io::Result<Self> {
         fs::copy(source.path(), path)?;
         let file = OpenOptions::new().read(true).write(true).open(path)?;
diff --git a/src/obicompactvec/src/colgroup.rs b/src/obicompactvec/src/colgroup.rs
index b1545e6..49ca477 100644
--- a/src/obicompactvec/src/colgroup.rs
+++ b/src/obicompactvec/src/colgroup.rs
@@ -126,8 +126,7 @@ pub fn eval_filter_mask(expr: &FilterMask, mat: &dyn MatrixGroupOps, n: usize) -
             b.freeze()
         }
         FilterMask::And(parts) => {
-            let mut b = TempBitVecBuilder::new(n)?;
-            b.not(); // initialise à tout-1 (tout passe)
+            let mut b = TempBitVecBuilder::new_ones(n)?;
             for part in parts {
                 let m = eval_filter_mask(part, mat, n)?;
                 b.and(m.view());
diff --git a/src/obicompactvec/src/tempbitvec.rs b/src/obicompactvec/src/tempbitvec.rs
index b8991df..8bbec16 100644
--- a/src/obicompactvec/src/tempbitvec.rs
+++ b/src/obicompactvec/src/tempbitvec.rs
@@ -56,6 +56,13 @@ impl TempBitVecBuilder {
         Ok(Self { builder, temp })
     }
 
+    pub(crate) fn new_ones(n: usize) -> io::Result<Self> {
+        let temp = TempDir::new()?;
+        let path = temp.path().join("data.pbiv");
+        let builder = PersistentBitVecBuilder::new_ones(n, &path)?;
+        Ok(Self { builder, temp })
+    }
+
     pub(crate) fn freeze(self) -> io::Result<TempBitVec> {
         let Self { builder, temp } = self;
         let vec = builder.finish()?;
diff --git a/src/obicompactvec/src/tests/colgroup.rs b/src/obicompactvec/src/tests/colgroup.rs
index 884450f..d1c7cf1 100644
--- a/src/obicompactvec/src/tests/colgroup.rs
+++ b/src/obicompactvec/src/tests/colgroup.rs
@@ -150,8 +150,7 @@ fn mask_with_all_ones_is_noop() {
     let dir = tempdir().unwrap();
     let mut v = PersistentCompactIntVecBuilder::new(4, &dir.path().join("v.pciv")).unwrap();
     v.set(0, 300); v.set(1, 1); v.set(2, 0); v.set(3, 42);
-    let mut mask = PersistentBitVecBuilder::new(4, &dir.path().join("m.pbiv")).unwrap();
-    mask.not();  // all bits → 1
+    let mask = PersistentBitVecBuilder::new_ones(4, &dir.path().join("m.pbiv")).unwrap();
     v.mask_with(mask.view());
     v.close().unwrap();
     let r = PersistentCompactIntVec::open(&dir.path().join("v.pciv")).unwrap();
diff --git a/src/obilayeredmap/src/layer.rs b/src/obilayeredmap/src/layer.rs
index 241feea..475bca7 100644
--- a/src/obilayeredmap/src/layer.rs
+++ b/src/obilayeredmap/src/layer.rs
@@ -106,9 +106,7 @@ impl Layer<()> {
         let presence_dir = layer_dir.join(PRESENCE_DIR);
         fs::create_dir_all(&presence_dir).map_err(OLMError::Io)?;
         let mut mb = PersistentBitMatrixBuilder::new(n_kmers, &presence_dir).map_err(OLMError::Io)?;
-        let mut col = mb.add_col().map_err(OLMError::Io)?;
-        col.not();
-        col.close().map_err(OLMError::Io)?;
+        mb.add_col_ones().map_err(OLMError::Io)?.close().map_err(OLMError::Io)?;
         mb.close().map_err(OLMError::Io)
     }
 }