Rats were fed on two deficient diets and were infected with N. brasiliensis larvae. They were autopsied at times ranging from 18 hours to 17 days after infection. The effect of the diets on worm burden, sex ratio of the worms and size of larvae and of worms were noted. Significantly more larvae reached the lungs and intestine in rats fed on glucose and filter paper (G.F.P.) and low protein (L.P.) diets than in controls, the highest numbers coming from G.F.P. rats. Neither the migration rate nor the development of the worms was accelerated by the deficient diets. G.F.P. and control rats lost larvae soon after their arrival in the intestine. Adult worms were lost from rats on the deficient diets before they were lost in significant numbers by controls; this was not due to the initially larger worm burdens in the former rats. G.F.P. and control rats lost the bulk of their infections by 17 days; L.P. rats retained about 50% of the worm burden they had on the fourth day. The effect of the deficient diets on the migratory and adult phase of N. brasiliensis, and on susceptibility and resistance is discussed.This work was done during the tenure of a Science Research Council Studentship. My thanks are due to Dr E. T. B. Francis for his helpful and critical supervision, and to Professor I. Chester Jones, in whose Department the work was carried out, for the facilities he provided.