Monthly day/night melatonin activity profiles were determined by radioimmunoassay over a 13-month period in the colonial anthozoan Renilla ko Β¨llikeri, and no daily rhythmic oscillation was found. Averaging those monthly values yielded a seasonal quantitative rhythm in both colonial and non-colonial tissues of this cnidarian, with spring and summer levels found to be four to five times higher than autumn and winter ones. The annual rise, which occurred in two successive Aprils, coincided with the first stages of sexual maturation in R. ko Β¨llikeri. Immunohistochemistry with a melatonin antibody raised in sheep revealed an exclusively neuronal distribution of melatonin-immunoreactivity (MEL-IR) in the endodermal septal filaments wrapped around gametophores, in endodermal walls of the rachis, and in the ectoderm of polyps. The MEL-IR ectodermal neurons shared many morphological features with serotonin-immunoreactive (5-HT-IR) neurons previously described in this animal but showed either weak or absent 5-HT-IR in double-labelling experiments. In contrast, MEL-IR and 5-HT-IR were strongly colocalized in endodermal neurons. These results indicate that melatonin is not a daily photoperiodic messenger but may instead act as a seasonal marker for reproduction in this cnidarian. We also provide the first evidence of a neuronal localisation of melatonin in an invertebrate, which suggests that melatonin may act as a neurotransmitter or neurohormone in the least evolved animals endowed with a nervous system.