Abstract 1. 1. Six sterols have been detected in Trypanosoma cruzi cultured on serum-free medium. Four of these are ergosterol, 22,23-dihydroergosterol, 7-dehydroporiferasterol and 7-dehydroclionasterol (or their C24 isomers from which they cannot be distinguished with the small quantities available). The remaining two sterols may be 24-methylene-7-dehydrocholesterol and 24-ethylidene-7-dehydrocholesterol but this has not been proven. 2. 2. In addition to these six sterols, cholesterol is also a major component of T. cruzi cultured in the presence of serum. 3. 3. Ergosterol (or its C24 isomer) was the only sterol detected in Crithidia fasciculata cultured in serum-free medium. When grown in the presence of serum the cells also contained cholesterol and 22,23-dihydroergosterol (or its C24 isomer).