Schistosomiasis was found in many agricultural workers from Yemen who are residing in the San Joaquin Valley, and a study was done to evaluate the public health hazard of this imported disease. If the necessary intermediate hosts are present, a local focus of infection could be established. Numerous Biomphalaria obstructa snails collected in the vicinity of the Avenue 82 irrigation canal near Oasis, Riverside County, showed no evidence of natural schistosome infection. Laboratory-reared offspring of these snails were exposed to miracidia of Schistosoma mansoni originating from Yemen immigrants and to miracidia from a standard laboratory strain of Puerto Rican origin obtained from the National Institutes of Health. All exposures of California B. obstructa were made with a laboratory stock of highly susceptible B. glabrata snails also obtained from NIH as controls. Although miracidia penetrated California snails no evidence of infection was detected whereas the B. glabrata controls showed normal, mature infections with numerous cercariae.There appears little likelihood that B. obstructa can serve as intermediate host for schistosomiasis.