Representatives of an undescribed, haploid, heterothallic yeast species, Pichia euphorbiae, have been recovered from insect-infested specimens of Euphorbia ingens. The new species is considered to be another representative of the apparently related group of heterothallic species, comprising Pichia rhodanensis, Pichia wickerhamii, Pichia veronae, Pichia amylophila, Pichia mississippiensis and Pichia meyerae. While no mating response was observed in interspecific mixtures of the mating types of the new species with the mating types of the first five mentioned species, a sexual response, manifested by the formation of non-sporulating zygotes, was nevertheless detected with the mating types of P. meyerae. Prototrophic hybrids obtained by crossing auxotrophic mutants of the mating types of P. euphorbiae and P. meyerae were found to be only partial recombinants. A description of the new species is given.