Many responses of the zygomycete fungus Phycomyces blakesleeanus are mediated by blue light, e.g. the stimulation of β-carotene synthesis (photocarotenogenesis) and the formation of fruiting bodies (photomorphogenesis). Even though both responses have been described in detail genetically and biophysically, the underlying molecular events remain unknown. Applying a pharmacological approach in developing mycelia, we investigated the possible involvement of heterotrimeric G proteins in the blue-light transduction chains of both responses. G protein agonists (guanosine triphosphate analogues, cholera toxin, pertussis toxin) mimicked in darkness the effect of blue light for both responses, except for cholera toxin, which was ineffective in increasing the β-carotene content of dark-grown mycelia. Experiments combining the two toxins indicated that photocarotenogenesis could involve an inhibitory G protein (G_i) type, whereas photomorphogenesis may depend on a transducin (G_t type)–like heterotrimer. The determination of the carB (phytoene dehydrogenase) and chs1 (chitin synthase 1) gene expression under various conditions of exogenous challenge supports the G protein participation. The fluctuations of the time course measurements of the carB and chs1 transcripts are discussed.