The fat sand rat Psammomys obesus is the main reservoir host of cutaneous leishmaniasis in North Africa and western Asia. In order to understand better the transmission of this disease to humans, a longitudinal study of a population of the rodent was carried out in Tunisia. Rodents were collected in 10 trapping sessions between May 1995 and January 1997, using mole traps and cage traps. The structure of the population was established from eye lens weights and the reproductive state was determined on dissection. Reproduction took place continuously from autumn 1995 to spring 1996 with sexual activity rates from 60–65% in females and 23–100% in males. The mean litter size rose from 3.6 in September to 6.1 in March. Reproduction ceased completely in the summer of 1996, and did not start again till January 1997, following an apparent failure in the autumn. Animals born early in the breeding season bred at an early age, but those born later delayed breeding till the following season. At most times the population clearly consisted of two cohorts, one born in the most recent breeding season, and one born in the preceding one. In response to the reproductive cycle, population numbers showed an increase in winter and a decline in summer. The peak observed in March 1996 was due to the sexual activity of multiparous females and young breeding females. At this time there was a female bias in the sex ratio. In a winter dry period, favourable habitat for breeding was confined to an area in which several species of Chenopodiaceae were associated. Sexual activity in females was clearly correlated with rainfall and rain days, with a 1 month delay (r= 0.78; P= 0.007). Male sexual activity was independent of weather conditions. This result is comparable with other rodents of Sahelian or tropical conditions. The increase in fertility was unrelated to age and is discussed in the light of food availability as a proximate factor. The persistence of the parasite causing cutaneous leishmaniasis in host populations is discussed.