Variation in daily egg counts, and in counts for samples of the same stool, was investigated in children with Schistosoma mansoni and Ascaris lumbricoides infections, using a thin smear technique. The results indicated that counts vary according to the stool consistency; drier stools from the same person producing up to seven times greater counts than wet ones. The sensitivity of the technique was tested in schoolchildren living in an endemic S. mansoni area, and found to detect all infections with intensities of greater than 50 eggs per gram of stool, after examination of only one slide containing 40mg stool. The sensitivity varied with the intensity of infection. The relevance of these findings to diagnosis in field situations is discussed.