Zoosporogenesis was observed in the algal symbiont (Trebouxia gelatinosa) of Parmelia caperata. This is the first detailed report of this phenomenon in a Trebouxia species and, more importantly, in lichens, where zoosporogenesis in this alga is usually suppressed and the arrested zoospores form aplanospores. Observations of zoospore formation by algal cells within the natural thallus and in thallus fragments incubated on a mineral medium suggest that zoospores are released within thalli and may escape from the thallus to form free-living microcolonies. These colonies potentially could unite with hyphae derived from spores of either the same, or genetically different mycobionts (fungal symbionts) to establish new lichen associations, as has been shown in laboratory studies. Such natural resyntheses could be one cause for the observed heterogeneity characterizing fungi in many lichen populations. Other implications of free-living Trebouxia populations are discussed.