The aqueous extract from the cultural mycelium of Paecilomyces japonica showed cytotoxicity against several tumor cells including Jurkat, U937, HL-60, HepG2, BW5147.G.1.4, and NIH3T3. When the aqueous extract was fractionated by sequential organic solvent extractions using n-hexane and ethyl acetate, the ethyl acetate fraction appeared to contain the most cytotoxic activity, and the $IC_{50}$ values for various tumor cells were in the range from 1.5 to $10.0{;}mu extrm{g}/ml$. To elucidate the cellular mechanism underlying the induced cytotoxicity, the apoptotic DNA fragmentation along with the cell cycle proression was examined in Jurkat T cells following the ethyl acetate fraction treatment. In the presence of $2.5{;}mu extrm{g}/ml$ of the ethyl acetate fraction, apoptotic DNA fragmentation of the cells was detected within 1 h and increased upto 24 h in a time-dependent manner. Under the same conditions, a sub-G1 peak was detectable by flow cytometry. These results indicate that the cytotoxic effect of P. japonica on tumor cells is attributable to the induced apoptosis.