Ipsen, J. (Dept. Community Medicine, Univ. of Pennsylvania, Philadelphia, Pa. 19104) and P. Feigl. A biomathematical model for prevalence of trichomonas vaginalis. Amer. J. Epid., 1970, 97: 175–184.—A cervical cancer survey by Dubin and Dunn provided auxiliary data on trichomonas for over 38, 000 women. The pre-menopausal occurrence by age is assumed to follow a two parameter catalytic model: p = A/(A + B)[1− exp ( −(A + B)t)] where A b infection risk, B is recovery rate, and f is age in five-year units with origin at 12.5 years. Infection risk in Negro women was estimated to be three times greater than in White women; recovery rate was similar for the two groups. In post-menopausal women the prevalence could be described by an equation of the form: p = _p0 exp ( − C(z + W)) where z is time since menopause, W is lead or lag time measured from menopause, and _p0 is the prevalence, p₁ at the time z = −W. Recovery rates (B and C) for pre- and post-menopausal groups were similar and infection risk appears negligible after menopause. The magnitude of W appears to depend on whether menopause occurs naturally or artificially. All parameters were estimated through direct evaluation of the likelihood functions over appropriate grids.