Problem with bytea getBytes and setBytes

I am trying to use a bytea column to store binary data, but the getBytes
and setBytes methods aren't working as expected.
I suspect a strange escaping occuring.
I ran the following code :

// insert test value
PreparedStatement stmtInsert connection.prepareStatement("insert into
test_blob (colblob) values(?)");
byte testBytes[] = new byte[255];
for (int i=0;i<255;i++)
    testBytes[i]=(byte)(i-128);
stmtInsert.setBytes(1,testBytes);
int count = stmtInsert.executeUpdate();
stmtInsert.close();
stmtInsert = null;

// read and compare test value
PreparedStatement stmtSelect =connection.prepareStatement("select
colblob from test_blob");
ResultSet rs = stmtSelect.executeQuery();
while (rs.next()){
    byte resByte[]=rs.getBytes(1);
    for (int i=0;i<255;i++){
       if (resByte[i]!=testBytes[i])
          System.out.println("Problem with byte "+i + " "+resByte[i]+"
different from original " + testBytes[i] );
       }
    }
rs.close();

The results (truncated) :
Problem with byte 32 -62 different from original -96
Problem with byte 33 -96 different from original -95
Problem with byte 34 -62 different from original -94
Problem with byte 35 -95 different from original -93
Problem with byte 36 -62 different from original -92
Problem with byte 37 -94 different from original -91
Problem with byte 38 -62 different from original -90
...

Xavier Poinsard.