Ca2+ enhanced the transformation frequency of Thermoactinomyces vulgaris (stock no. 1278) of an auxotrophic strain by the chromosomal DNA isolated from a prototrophic strain (stock no. 1227). The number of transformants showed a marked increase with increasing concentration of CaCl2 upto 0.05 mM; and above this concentration, the transformation frequency decreased significantly. Antipsychotic drugs that are potent calmodulin inhibitors, like trifluoperazine and chlorpromazine, when applied in the concentration range of 0.01-0.04 mM along with optimal CaCl2 concentration to the cultures of the recipient cells, resulted in a significant inhibition in the frequency of Ca2+-stimulated transformation. The results of present investigation suggest the involvement of a Ca2+-dependent protein activator in the development of Ca2+-mediated competence, which could have played an important role in the enhancement of genetic transformation in this aerobic spore forming thermophilic actinomycete.