The population dynamics of the exotic earthworm Hyperiodrilus africanus was investigated in a secondary forest of the Natural Reserve of Lamto (Ivory Coast) over a period of 19 mo. The objectives were to assess seasonal abundance patterns and to determine the adaptive strategies of this species. Each month, 10 soil samples of 100×100×40 cm and 20 monolith samples of 25×25×30 cm were randomly excavated in a plot of 50×95 m and earthworms were extracted by both hand-sorting and wash-sieving methods. The results show significant inter-annual and seasonal fluctuations in population size. Three factors are likely to control population dynamics: (1) rainfall, (2) soil water content and (3) seasonality. The dry season appears to be the most important environmental factor that regulates population abundance when predation, density-dependent regulation and competition phenomena are ignored. Hyperiodrilus africanus exhibited an r strategy, suggesting a high ability to recover populations affected by drought.