Anders Kock: Characterization of stacks of principal fibre bundles is available on the address http://www.ml.kva.se/preprints/meta/KockThu_May_17_12_51_36.rdf.html Abstract: Stacks of principal fibre bundles are stacks whose total category has binary products, and an atlas. This holds on any basis category equipped with a class of descent morphisms (the notions of atlas, stack, etc. refer to this class.) (Note: my permanent e-mail address is unchanged, kock@imf.au.dk) Anders K.