Concentrated stool samples from 560 children, aged from two weeks to 10 years, from Idukki District, southern India, were examined to detect Cryptosporidium oocysts, using a phenol-auramin stain and fluorescent microscopy. Smears of stool samples were also stained by a modified Ziehl-Neelsen method and were examined by light-microscopy. One sample was collected from each of 266 children with diarrhoea and 294 age-matched children without diarrhoea. Cryptosporidium oocysts were detected in 4.5% of all stools, in 6% of diarrhoeal stools and in 3% of stools from healthy controls. Results of the study show that Cryptosporidium infection is common in children with diarrhoea in southern India, and there is also a high frequency of asymptomatic cyst-passers in this area.