With numarray 1.0 and Mandrake 10 i686 I get the following: >>> y=N.array([1,1,2,1],type="Float64") >>> y array([ 1., 1., 2., 1.]) >>> y.byteswap() >>> y array([ 3.03865194e-319, 3.03865194e-319, 3.16202013e-322, 3.03865194e-319]) >>> y.isbyteswapped() 0 Should this be 1? Thanks, Phil