Field observations spanning 1 year of the visits of two bee species--Tetraglossula bigamica (Hymenoptera: Colletidae) and Pseudagapostemon brasiliensis (Hymenoptera: Halictidae)--to the flowers of Ludwigia elegans (Onagraceae) evinced the existence of circadian and circannual components in the rhythms of activity both of plant and bees. In addition to morphological adaptations to the flower, the bees were temporally adjusted to each other and to the flower's opening and closing mechanisms. At the observation locality (Ribeirão Preto, São Paulo, Brazil--latitude 21 degrees-21 degrees 30S), the plant bloomed year-round. The flowers of the same population open simultaneously at the onset of the photophase, pollen and nectar being available almost immediately. Withering occurs in the afternoon of the same day. Although many bee species visited the flowers, T. bigamica and P. brasiliensis were the most frequent visitors, but their yearly rhythms were phased opposite to one another. Circadian rhythms were detected in pollen and nectar harvest. Comparisons of acrophases showed significant differences for males' and females' nectar collecting rhythm and for females' pollen harvest rhythm. As for the circannual component, acrophases of all rhythms occur at progressively later times from December to July when the migration is reversed until November. These data strongly suggest that the relationship between L. elegans and T. bigamica and P. brasiliensis is in accordance with a clearly temporal pattern and that the two bee species are temporally separated in nature.