We examined quantitative changes in biogenic amines in relation to effects of population density on growth and behavior in the cricket Gryllus bimaculatus, which were reared in two different densities, completely isolated solitary and 40 crowded insects. In the crowded animals, the rate of increase in body weight was slow, and also the day of imaginal moult was late, when compared to the isolated ones. Development and growth were thus suppressed in the crowded group. The body color of isolated ones was darker than that of crowded ones. Behavioral tests indicated that the isolated crickets were significantly more aggressive than the crowded ones. The levels of biogenic amines, their precursors and metabolites were examined in the brain, corpora allata, corpora cardiaca and frontal ganglion of isolated and crowded crickets using high-performance liquid chromatography with electrochemical detection. The brains of crowded crickets contained significantly higher amounts of octopamine, dopamine, and 5-hydroxytryptamine (5-HT) than those in the isolated ones. On the contrary, the level of N-acetyldopamine in the isolated crickets was significantly higher than that in the crowded ones. In crowded animals the corpora allata contained significantly higher levels of 5-HT, and the corpora cardiaca contained more octopamine. These results indicate that high population density affects aminergic systems which in turn modulate various biological events, such as development, growth and behavior.