  We present an experimental and theoretical investigation of spontaneous
pattern formation in the transverse section of a single retro-reflected laser
beam passing through a cloud of cold Rubidium atoms. In contrast to previously
investigated systems, the nonlinearity at work here is that of a 2-level atom,
which realizes the paradigmatic situation considered in many theoretical
studies of optical pattern formation. In particular, we are able to observe the
disappearance of the patterns at high intensity due to the intrinsic saturable
character of 2-level atomic transitions.
