The aggressive use of praziquantel to combat schistosomiasis in Egpyt raises concern about the possible emergence of resistance. Eggs from Egyptian patients with praziquantel-resistant infections (not cured by 3 doses of praziquantel) have been used to establish infection-specific schistosome isolates in mice. The response of these worms to the drug was observed in vitro, in order to determine if the isolates obtained from these resistant infections were, in fact, less responsive to praziquantel. One of the hallmark effects of praziquantel on schistosomes in vitro is a disruption of the worm's outer surface, the tegument. Here, praziquantel-induced tegumental damage is observed in 3 distinct isolates, 2 derived from resistant infections and 1 from an infection cured by a single dose. The isolates from the resistant infections were less susceptible to praziquantel-induced tegumental damage in vitro, suggesting that the worms are in some way less responsive to the drug.