diff --git a/noodles-bam/src/io/reader/header.rs b/noodles-bam/src/io/reader/header.rs
index c9fd80508..e8dc7470a 100644
--- a/noodles-bam/src/io/reader/header.rs
+++ b/noodles-bam/src/io/reader/header.rs
@@ -1,3 +1,4 @@
+mod magic_number;
 mod reference_sequences;
 mod sam_header;
 
@@ -6,7 +7,7 @@ use std::io::{self, BufRead, Read};
 use byteorder::{LittleEndian, ReadBytesExt};
 use noodles_sam::{self as sam, header::ReferenceSequences};
 
-use self::reference_sequences::read_reference_sequences;
+use self::{magic_number::read_magic_number, reference_sequences::read_reference_sequences};
 use crate::MAGIC_NUMBER;
 
 struct Reader<'r, R> {
@@ -45,22 +46,6 @@ where
     read_header_inner(&mut header_reader)
 }
 
-fn read_magic_number<R>(reader: &mut Reader<R>) -> io::Result<()>
-where
-    R: Read,
-{
-    let magic_number = reader.read_magic_number()?;
-
-    if magic_number == MAGIC_NUMBER {
-        Ok(())
-    } else {
-        Err(io::Error::new(
-            io::ErrorKind::InvalidData,
-            "invalid BAM header",
-        ))
-    }
-}
-
 fn read_header_inner<R>(reader: &mut Reader<R>) -> io::Result<sam::Header>
 where
     R: Read,
@@ -161,29 +146,6 @@ mod tests {
         None => unreachable!(),
     };
 
-    #[test]
-    fn test_read_magic_number() -> io::Result<()> {
-        let mut src = &b"BAM\x01"[..];
-        let mut reader = Reader::new(&mut src);
-        assert!(read_magic_number(&mut reader).is_ok());
-
-        let mut src = &[][..];
-        let mut reader = Reader::new(&mut src);
-        assert!(matches!(
-            read_magic_number(&mut reader),
-            Err(ref e) if e.kind() == io::ErrorKind::UnexpectedEof
-        ));
-
-        let mut src = &b"MThd"[..];
-        let mut reader = Reader::new(&mut src);
-        assert!(matches!(
-            read_magic_number(&mut reader),
-            Err(ref e) if e.kind() == io::ErrorKind::InvalidData
-        ));
-
-        Ok(())
-    }
-
     #[test]
     fn test_read_header() -> io::Result<()> {
         let mut data = Vec::new();
diff --git a/noodles-bam/src/io/reader/header/magic_number.rs b/noodles-bam/src/io/reader/header/magic_number.rs
new file mode 100644
index 000000000..c6f031b42
--- /dev/null
+++ b/noodles-bam/src/io/reader/header/magic_number.rs
@@ -0,0 +1,48 @@
+use std::io::{self, Read};
+
+use super::Reader;
+use crate::MAGIC_NUMBER;
+
+pub(super) fn read_magic_number<R>(reader: &mut Reader<R>) -> io::Result<()>
+where
+    R: Read,
+{
+    let magic_number = reader.read_magic_number()?;
+
+    if magic_number == MAGIC_NUMBER {
+        Ok(())
+    } else {
+        Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            "invalid BAM header",
+        ))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_read_magic_number() -> io::Result<()> {
+        let mut src = &b"BAM\x01"[..];
+        let mut reader = Reader::new(&mut src);
+        assert!(read_magic_number(&mut reader).is_ok());
+
+        let mut src = &[][..];
+        let mut reader = Reader::new(&mut src);
+        assert!(matches!(
+            read_magic_number(&mut reader),
+            Err(ref e) if e.kind() == io::ErrorKind::UnexpectedEof
+        ));
+
+        let mut src = &b"MThd"[..];
+        let mut reader = Reader::new(&mut src);
+        assert!(matches!(
+            read_magic_number(&mut reader),
+            Err(ref e) if e.kind() == io::ErrorKind::InvalidData
+        ));
+
+        Ok(())
+    }
+}