Objective: To measure types of sex role prevalence in common and risk-related behaviours among gay men for modelling HIV transmission. Design: Cohort study of 385 homosexually active men recording sexual diaries over 1-month periods. Methods: Measures of incidence of behavioural sex roles for masturbation, fellatio, anal intercourse and anilingus by relationship type, derived from 1-month sexual diary data. Results: Low behavioural role rigidity for masturbation and fellatio, but higher rigidity for anal intercourse and anilingus. Participants with no regular partner showed a relatively low frequency of anal intercourse, whereas those in closed relationships showed a high frequency. Conclusion: Although anal intercourse shows a certain degree of behavioural role rigidity, this rigidity is not large enough to conclude that gay men exclusively engage in either an active or a passive role. Typical rates for exclusive active and passive roles for anal intercourse during the month the diaries were recorded were in the range of 12-15%; the dual role was significantly higher. Keywords: Homosexual behaviour, HIV transmission, modelling, behavioural role separation, sexual diaries.