The protozoa Leishmania undergo morphological and biochemical transformation from the promastigote to the amastigote form during their life cycle. To characterize this transformation process, we constructed a cDNA library for the promastigote stage of Leishmania major and used differential cDNA hybridization to identify cDNA sequences expressed at different abundance in promastigotes or amastigotes of L. major. P100/11E is a single copy gene whose 1600-nucleotide mRNA is enriched in promastigotes. P101/10 is a repeated gene whose 3300-nucleotide transcript is enriched in amastigotes. These developmentally regulated genes are not linked in the genome of L. major and are located on separate chromosome bands. The abundance of the P101/10 transcript increases severalfold during the transformation process at 37 degrees C in vitro and is thermally induced within 3 h after transfer of promastigotes from 27 to 37 degrees C. Examination of beta-tubulin gene expression showed that the relative abundance of the 3400-nucleotide beta-tubulin RNA is decreased at 37 degrees C in vitro. Our results indicate that the expression of two developmentally regulated genes of L. major is controlled at the level of mRNA abundance and provide direct evidence that thermal induction plays a general role in regulating gene expression in Leishmania.