The etiology of travelers' diarrhea was studied in 579 adult Finnish tourists participating in two packaged tours to Morocco in the winter (n = 233) and fall (n = 346) of 1989. A research team accompanied the travelers, and a laboratory for enteric pathogens was established in Agadir. At least one pathogen was found in 62% of the 60 diarrhea cases in winter and in 58% of the 111 diarrhea cases in fall. Multiple pathogens were found less often in winter (8%) than in fall (21%, P less than .05). Campylobacter strains were the leading cause of travelers' diarrhea in winter, found alone or with other pathogens in 28% of the cases (but in only 7% in fall), whereas enterotoxigenic Escherichia coli (ETEC) was the most common pathogen in fall, present in 32% of the cases (8% in winter). Both differences are highly significant (P less than .001). Salmonella enterica was almost as common as ETEC in fall (25% of diarrhea cases) but rare in winter (10%, P less than .05). Thus, the etiology of travelers' diarrhea varied according to the season in the same tourist destination. This finding has relevance to both antimicrobial treatment and prophylaxis.