We studied the larval distribution and composition of Anopheles arabiensis Patton, An. gambiae s.s. Giles, and its forms, among local habitats; and their association with the adults between these habitats in Banambani village, Mali during the mid-rainy seasons of 1997-1999. For species and form identification we used polymerase chain reaction (PCR) and PCR-restriction fragment-length polymorphism (RFLP). Differences among species in the distribution of larvae were observed in 1998, but not in 1997 or 1999, although they were on the borderline of statistical significance. Differences among the M and S molecular forms were statistically significant in 1999 when rainfall was high, but not in the two prior, drier sampling periods. Combining all information into the Fisher multiple comparisons test, there were statistically significant differences between species and molecular forms during the 3-yr study period. Hybrid larvae between the M and S forms were observed (0.57%), the first such observation to our knowledge. In spite of differences among larval distribution, no differences of adult species composition were observed among habitats. Factors that influence the distributions of An. gambiae larval populations are discussed.