Heavy outbreaks of migratory locusts, Locusta migratoria migratorioides, were observed in southwestern Chad and northern Cameroon from September to November 1997. For many years, similar outbreaks were a recurrent phenomenon in this region, with the last one occurring in 1989. An analysis of rainfall distribution from 1986 to 1997, was undertaken according to the ecological requirements of this locust. To assess the possibilities of good breeding and gregarisation, the monthly position of the areas receiving an optimal rainfall for this species (50 to 100 mm/month) was studied. The following parameters, assumed to be key factors, were considered: northern limit of the optimal area in May and June, northern limit of the area with excessive rainfall (>l00 mm/month) in June, maximal northern limit of the optimal area during the rainy season, maximal northern limit of the area where the rainfall is excessive, duration of optimal pluviometric conditions around Lake Chad during the rainy season. This study showed that the outbreaks were strongly linked to the extent and distribution of rainfall during the 1988-1989 and the 1996-1997 periods. A simple index, calculated on the basis of rainfall data, was correlated with outbreak years. It appeared that two successive years of suitable conditions are necessary to induce an outbreak.