The total number and distribution of schistosomula of Schistosoma mansoni and S. haematobium in all tissues and organs of mice from infection to 14–27 days was determined by compressed tissue autoradiography. Attrition of schistosomula, manifested as a decrease in the number of autoradiographic foci, was observed in organs other than the liver. Attrition commenced about 2 days after cercariae entered the skin, and conformed to a single exponential function with a rate constant (± standard error) of 7·0±0·5%/day for S. mansoni and 3·2±0·7%/day for S. haematobium. The temporal distribution of schistosomula of S. mansoni and S. haematobium differed quantitatively. In the case of S. mansoni, concomittant with a decrease in skin counts, the lung curve rose rapidly to a peak centred on day 6 and thereafter decreased more or less parallel to the total body curve. Significant accumulation in the liver was not observed until day 7, whereupon liver counts rose steadily to a plateau that extended from about day 14 to the end of the experiment and approximated the number of adult worms recovered from the hepatic portal vessels on day 42. A maximum of 26% and mean of 12% of all foci in the body were counted on autoradiograms of tissues other than the skin, lung and liver. The pelt averaged 14% of the body weight yet schistosomula were detected only in the area initially exposed to cercariae. The eviscerated carcass averaged 54% of the body weight yet contained only 0·8% −3·4 % of the schistosomula during the period of accumulation in the liver. Between day 6 and day 14, the ratio of schistosomula in the pulmonary circulation to schistosomula in the systemic circulation did not remain constant, as would be the case if schistosomula circulated passively and randomly, but instead displayed a statistically significant decrease from 0·92 and 0·85. For these reasons, it was considered unlikely that schistosomula had circulated randomly and repeatedly through the pulmonary and systemic circulations and entered the hepatic portal system by chance, as hypothesized by Miller & Wilson (1980). Instead it was considered more probable that schistosomula migrating from lungs to liver had followed a directed path through intervening vessels (Kruger, Heitman, van Wyk & McCully, 1969) or tissues (Wilks, 1967). Schistosoma haematobium distribution patterns differed from those of S. mansoni in slower movement of schistosomula from skin, their peak accumulation in lungs at about day 8 followed by a low rate of decrease, and a very low liver plateau which approximated the number of adult worms recovered from the hepatic portal vessels on day 42.