Abstract. Protothecosis and chlorellosis are sporadic algal diseases that can affect small ruminants. In goats, protothecosis is primarily associated with lesions in the nose and should be included in the differential diagnosis of causes of rhinitis. In sheep, chlorellosis causes typical green granulomatous lesions in various organs. Outbreaks of chlorellosis have been reported in sheep consuming stagnant water, grass from sewage-contaminated areas, and pastures watered by irrigation canals or by effluents from poultry-processing plants. Prototheca and Chlorella are widespread in the environment, and environmental and climatic changes promoted by anthropogenic activities may have increased the frequency of diseases produced by them. The diagnosis of these diseases must be based on gross, microscopic, and ultrastructural lesions, coupled with detection of the agent by immunohistochemical-, molecular-, and/or culture-based methods.