The in vivo effects of a range of concentrations of praziquantel (10, 25, 50, 100, 200 and 500 mg/kg body weight of mouse) on the tegumental surface of adult Schistosoma mansoni were studied using scanning electron microscopy. Worms were recovered from mice at 1 and 4 h post-treatment. In general, irrespective of the dose level, male worms exhibited more pronounced and extensive surface alterations which included surface bleeding, swellings, wrinkling and constrictions and surface lesions, particularly on the spined tubercles. In male worms, in particular, the number of worms exhibiting damage and the amount of tegumental surface damage depended, mainly, on the concentration of praziquantel, although, at any given dose level, the extent of the damage increased with time post-treatment.