Three age groups of Bulinus natalensis, immature, mature but not yet egg-laying and mature egg-laying, were infected with miracidia of Schistosoma margrebowiei. The growth of infected, exposed but showing no signs of infection, and uninfected control groups, were examined at weekly intervals for ten weeks post exposure. Snails exposed to infection when immature, even in the group where no patent development ensued, showed a statistically significant reduction in growth and this was evident as early as 2–5 weeks post exposure. When infection occurred at the stage prior to egg-laying a significant reduction in growth was seen but only in the group which developed a patent infection. This was also the case for the egg-laying group although the reduction in growth was only significant in the final three weeks of the experiment. Thus in all three age groups, growth rate was reduced and the infected snails were significantly smaller at the end of ten weeks compared with the controls. The reasons for these effects are discussed.
