Plasmodium vivax and P. falciparum epidemiology were studied for parasitological and entomological samples collected during the period 1989 and 1990, respectively, from Gambella, South West Ethiopia. Of the total population examined (n = 1091), 147 (13.5%) were found to be positive for malaria parasites. Prevalence rates among males and females were 13.8% and 13.1%, respectively. Differences in the prevalence rates of malaria in the eleven villages were observed, the highest (33.3%) being in Ukuna 2 and the lowest (3.9%) in Ukuna 22. The dominant species of malaria found were both P. falciparum and P. vivax. 88.9% and 11.1% of the malaria cases of the general population were due to these parasites, respectively. It was also recognized that P. falciparum and P. vivax were prevalent in 81.6% and 18.4% of the Anuak population, respectively. The mosquito species responsible for malaria transmission were the indoor-resting A. gambiae s. l. and A. pharoensis. The parasite infection rates of these species were 0.76% and 0.46% and they were found to be the exclusive vectors of P. falciparum and P. vivax, respectively. The present findings are not in accord with the study results previously reported twenty years ago by Armstrong (1972) and Krafsur (1971). The most probable contributing factors for such switch of malaria transmission patterns were, the rehabilitation and resettlement programmes and agricultural activities undertaken in Gambella for the past 10 years that may have brought changes of the socio-economic situation and environmental factors.