Gene expression and its regulation was studied in the rat tapeworm, Hymenolepis diminuta, during strobilization. RNAs extracted from the different developmental stages of the parasite were translated in vitro in a rabbit reticulocyte lysate system. Two-dimensional patterns of the translational products were compared with the 2-dimensional patterns obtained by metabolic labelling with [35S]methionine. The results indicated a post-transcriptional regulation of gene expression during strobilization in this parasite. Gene expression and its regulation was also studied in H. diminuta obtained from 'non-permissive' hosts (mice) and immune suppressed mice and compared with the parasites of the same age obtained from 'permissive' rats hosts. The 2-dimensional patterns of the in vitro translation products, obtained by translating the RNA of different groups of parasites, were compared with the patterns of gene products obtained from parasites using [35S]methionine metabolic labelling. The results indicated a massive post-transcriptional regulation of gene expression, the latter inhibited in parasites obtained from normal mice, but not in immune suppressed mice.