SUMMARY : Ageing, chilling, and media containing suitable concentrations of glycine were found to stimulate the production of L-forms in cultures of Haemophilus pertussis. Stained with Giemsa these L-forms are morphologically similar to the L-forms of many other bacilli produced by similar stimuli. Serologically the L- and bacillary forms appear to be related. The suggestion is put forward that the small round red form is a 'resistant' form of H. pertussis and reverts to the rod by passing through the stages characteristic of the L-cycle. Information on the structure and morphological variation of Haemophilus pertussis is very limited when compared with that of many other Gram- negative species. One of the interesting attributes of these other species is their ability to produce large bodies or L-forms (according to the terminology of Klieneberger, 1935) when subjected to suitable stimuli. This investigation was undertaken, therefore, to determine whether H. pertussis was similar to other Gram-negative bacilli in this morphological feature. MATERIALS AND METHODS