A hematology and parasitological survey was conducted in Bayaney, Hatillo, a rural community located in north-central Puerto Rico. A surprising finding was the comparatively high proportion of eosinophilia observed (43 of 83 individuals: 52%). Of these same 83 individuals 16 were found positive for intestinal helminth eggs using the modified Ritchie formol-ether concentration method, which uses 1 gram of feces. When eosinophilia was in the 5 to 10% range, 35% were also found to be infected with one helminth parasite. When eosinophilia was in the 10 + % range, 75% of the individuals were also found to be infected with one helminth parasite. The helminth eggs or larvae found were: hookworm (n = 8), Strongyloides stercoralis (2), Trichuris trichiura (5), and one individual with a combined infection of hookworm and Trichuris. Fifteen of the 16 (94%) individuals with intestinal helminth infections had eosinophilia ranging from 5 to 40%. Of a subgroup of 5th grade school children, 11 of 35 (31%) had eosinophilia ranging from 5 to 17%, but only two of these also had helminth eggs (Trichuris). No intestinal protozoan parasites were found. Although all infected individuals were apparently asymptomatic, they were informed of the results of the survey and contacted for additional medical examination and treatment, where applicable. An ELISA for anti-S. mansoni egg antibodies using a cationic antigen fraction denoted CEF-6 was done with the serum of 44 individuals of which 20 were a high risk group because of frequent water contact. Only one (2%) was found positive by serology and this one was negative for S. mansoni eggs by coprology. Additional studies are warranted to elucidate the etiology of eosinophilia in the absence of helminth infection.