A study was conducted during 1990-96 in a semi-evergreen forest dominated by Sal trees ( Shorea robusta ) located at Ramnagar, south Nepal, to assess the ecological conditions, feeding and ranging behaviour and physical condition of female Hanuman langurs ( Presbytis entellus ), to analyse the relationships with the timing of reproductive events. The climate was dominated by the summer monsoon lasting from May to September. The availability of plant phytophases was markedly seasonal, with fruiting as well as flushing of the 6 main food plants during the monsoon. Feeding and ranging behaviour of the females varied with food availability and its distribution. High dietary quality was only obtained during a short period from May to July. Physical condition of the females varied in accordance with dietary quality and energy expenditure, with a good physical condition during the second half of the year. Births occurred from January until June, with most births around the energetically worst period of the year. The results indicate that, in contrast to several other non-human primates, most females of this population gave birth too early to meet the optimum lactation requirements. It is suggested that the seasonality and timing of births may be the result of a regular seasonal shortfall of resources influencing the ability of the females to ovulate and to conceive.