During a 12-month period, feces from 780 persons from the Townsville region were evaluated by the Kinyoun acid-fast strain, and 36 (4.6%) immunocompetent patients were found to have Cryptosporidium oocysts. Twenty-five index cases were identified; 13 (8.6%) cases from 151 patients were from Palm Island, an isolated Aboriginal community in the wet tropics and 12 (1.9%) cases from 629 patients were from the dry tropics of Townsville. All 11 secondary cases were associated with a person-to-person outbreak in the nursery of a Townsville day-care centre. Infection occurred mainly in two distinct age groups: the under five-year-old (27 cases), and the 25 to 35-year-old (six cases). A prodrome of dry cough, rhinorrhea and vomiting often preceded symptoms of fever, weight loss, abdominal pain, persistent cough and vomiting, and acute diarrhea with frequent, non-bloodstained, watery, mucous stools. Although 13 patients were hospitalised because of their illness, the infection was self-limiting and all 36 patients recovered with symptomatic treatment. Cryptosporidium was the third most commonly identified enteric pathogen after Rotavirus and Giardia. Infection did not appear to depend on seasonal variation and no animal or environmental sources of infection were identified. Cryptosporidiosis in immunocompetent persons is endemic and common in North Queensland and routine investigations for this parasite in symptomatic patients are warranted.