Seasonal fluctuations, phenology and diet of Misumenops argenteus (Araneae, Thomisidae) on Trichogoniopsis adenantha (Asteraceae) were investigated in the Serra do Japi, southeastern Brazil, over a 2 year period. This spider population increased at the beginning of the rainy season, reaching a peak in March, and decreased in May, reaching its lowest density in the cold/dry season. In the rainy season (December–May), most of the individuals were in the young or juvenile phase (3^rd–6^th instars). The spiders reached adulthood between the end of the cold/dry season and the beginning of the hot/rainy season. Analysis of temporal displacement (with up to a 3 month delay) detected a one month delay between the blooming period of T. adenantha and the beginning of the rainy season. The number of arthropods (potential prey of M. argenteus) on the plants increased concomitantly with the increase in the number of reproductive branches. The M. argenteus population also increased numerically 2 months after the rise in arthropod density. These results indicate that the spiders require time to respond to changes in environmental conditions. Of the 595 spiders examined, 76 (12.8%) had prey. Prey items included arthropods belonging to several guilds, but spiders showed a preference for wingless prey or prey that remained on the branches for longer periods of time.