Cytology and development ofEntyloma thirumalachariPavgi and Singh, parasitic onBlumea oxydontaDC. have been described. During teliospore germination, the diploid nucleus migrates into and divides in the promyceliuin. The first division is reductional followed by an equational one. Some of the meiotic divisional stages were observed and the haploid chromosome complement determined to ben= 6. Several aberrations in teliospore germination and nuclear behavior were observed and their repercussions discussed. Sporidial development and their conjugation, parasitic mycelium and formation of clamp connections, and development of teliospores in the host tissue have been studied. The fungus possesses a diplobiontic rhythm of life cycle, which is well adapted for its effective dissemination and survival under the ecological conditions to which the host is subjected.