The research is financed by the Directorate General of Higher Education, Ministry of Education and Culture the Republic of Indonesia in fiscal year 2012. Abstract Fusarium wilt disease caused by Fusarium oxysporum f.sp. lycopersici is one of important diseases that potentially cause significant yield losses on tomato. There is no measure available to control the disease effectively. This study was done to isolate the antagonistic Pseudomonas sp. from rhizospeheres of plants of Leguminosae and Solanaceae and to determine their antagonistic activity against F.oxysporum f.sp.lycopersicum. Biochemical test and molecular analysis of the 16S rRNA gene were done to determine the species of Pseudomonas sp. There are three isolates of Pseudomonas sp. were obtained in this study namely isolates KtS1, TrN2, and TmA1. Results of 16S rRNA gene analysis showed that all three isolates had more than 99% similarity to P. alcaligenes. These isolates showed strong antagonistic activity against F. oxysporum f.sp. lycopersici with inhibitory activity more than 80%. Application of bacterial suspension of P. alcaligeneseffectively suppressed the disease incidence of Fusarium wilt on tomato. Keywords : antagonistic activity, Pseudomonas alcaligenes, Fusarium wilt disease
