The variability within schistosome populations was explored using mixed populations of cercariae from multimiracidial snail infections and individual clones of Schistosoma mansoni cercariae obtained from monomiracidial snail infections. We investigated the heterogeneity between different clones of S. mansoni with respect to infectivity and metabolism. One difference between clones of cercariae was found in the recovery of adult worms from Balb/C mice. Recovery of adult worms was greater after infections with a mixed population than with a clonal population. To investigate some biochemical features of individuals in clones or mixed populations, the uptake of [35S]methionine into individual parasites and their membrane proteins was measured. Isoelectric focusing of a soluble membrane fraction: the frozen–thawed supernatant extracted from individual clones, showed the presence of proteins of isoelectric point between 7.2 and 8.2 in all clones. These proteins were less labelled with [35S]methionine in the clones than in the mixed population. It was concluded that basic proteins are synthesized by all clones and in the mixed population but at different rates. Differences in the rate of incorporation of [35S]methionine into the surface membranes of schistosomula and adult worms derived from individual clones are reported. In addition, a direct correlation between the percentage of recovery of adult worms from mice infected with individual clones of S. mansoni and the rate of incorporation of [35S]methionine into schistosomula of these particular clones was observed. It is suggested that the high rate of metabolism shown by an individual clone may account for the enhanced survival of the cercariae derived from that clone during penetration of the skin and migration through the vertebrate host. In order to examine individuals in a population of schistosomula, from a clone or mixed population, the lysosome-specific fluorescent probe LysoTracker DND-99 was used to label the parasites and quantitative fluorescent measurements were made on individual parasites. There were significant differences between clones and a mixed population. Furthermore, the variation between individuals from a mixed population was greater than from that in any clone, just as was found in the infectivity studies. Freshly transformed schistosomula of individual clones labelled with the LysoTracker DND-99 showed less variations in the quantitative uptake of the dye within a single clone when compared to the mixed population. We conclude that for any biochemical and biological parameter, a population of cercariae consists of individuals showing a wide range of values, with a much greater range in a mixed population. This variability is likely to have great relevance for infectivity of the final host and the efficacy of drugs and the immune system.