Objective: To determine the prevalence and seasonal variation, and to assess the clinical manifestations and treatment of blastocystosis in Libyan patients. Methods: Three thousand six hundred and forty five stool samples were screened for Blastocystis hominis using normal saline and iodine solution preparations. The clinical features of 08 patients were described, in whom B. hominis was the only parasite isolated. Fifty symptomatic patients were treated with 500 mg metronidazole daily for 7 days and their stools were re-investigated for B. hominis. Results: B. hominis was found in 969 (26.58 %) of 3645 stool specimens examined. The infection of B. hominis was significantly more (p < 0.05) in summer than in winter over a three year period. In a prospective study of 08 patients, the most common symptoms with stools positive only for B. hominis were diarrhoea (84.94 %), abdominal pain (66.66 %), flatulence (7.20 %) and vomiting (6.2 %). High concentration of B. hominis cells were found more in symptomatic patients than asymptomatic ones (9.20 cells per 40 X field versus 4.06 respectively) with statistically significant differences (p < 0.00). Patients with B. hominis responded to metronidazole and were fully cured after 7 days. Conclusion: The occurrence of B. hominis infections in outpatients are probably related to weather conditions, with the suggestion that the hot, dry weather of the Sebha region favors the development and transmission of this organism. B. hominis infections might have a role in some pathological conditions, resulting in gastrointestinal symptoms.