The aim of this research was the analysis of the possible antagonistic effect of Penicillium oxalicum over the pathogen rice fungus A. alternata under different conditions of temperature, water activity and culture media. The macroscopic study of the dual growth revealed that according to the Index of Dominance P. oxalicum was more competitive that A. alternata at 25ºC whereas at 15ºC was this species. Microscopic analysis showed that P. oxalicum was a mycoparasite of A. alternata at all conditions tested. The antagonist penetrated into A. alternata and disintegrated its conidiophores and conidia. The results suggests that P. oxalicum may be a possible biological control agent of the rice pathogens in a future.