To examine the potential of antimicrotubule drugs for treating Pneumocystis carinii infections, and to learn more about this unusual organism on a molecular level, we are studying its tubulin genes. A 0.3 kbp fragment of the P. carinii beta-tubulin gene was amplified by the polymerase chain reaction. Sequence analysis of this DNA revealed that P. carinii beta-tubulin is most closely related to those of the fungal molds. Consistent with these results, P. carinii growth in vitro was sensitive to the antifungal benzimidazoles benomyl and carbendazim.