THE fate of the sporozoites of Plasmodium berghei in rodent hosts has been the subject of much research and speculation and has drawn the attention of malariologists from, the early days of the discovery of this parasite¹. For it was evident to all that this useful and easily accessible Plasmodium, an instrument of great promise in biological and medical research, could not achieve or claim true distinction so long as part of its life-cycle remained unmapped and obscure. Early attempts to elucidate the primary development of this parasite were, according to C. G. Huff, unsuccessful (personal communication). Investigations were greatly hampered by the necessity of obtaining sporozoites from wild-caught Anopheles dureni from the forest galleries of Katanga and in view of the early inability to infect laboratory-bred anopheline species under experimental conditions². The demonstration early in 1964 of successive cyclical transmissions of P. berghei by the bite of experimentally infected Anopheles quadrimaculatus in mice, hamsters and tree rats³ has brought new interest and a renewed attempt to solve the problem of the existence of a primary tissue phase of P. berghei. We report here some preliminary results of these investigations and our findings of pre-erythrocytic stages of development of this parasite.