A stress-induced ''mycosome'' phase of Aureobasidium pullulans consisting of minute reproductive propagules that may revert directly to walled yeast cells is described. Mycosomes detected by light-and electronmicroscopy reproduce within senescent plant plastids, and display three developmental pathways: wall-less cells (protoplasts), yeast cells, or membrane-bounded spherules that harbor plastids. Widespread in plant and algal cells, mycosomes are produced by both ascomycete and basidiomycete fungi. Keywords Aureobasidium pullulans AE Fungi AE Plastids AE Mycosomes AE Someangia rust brown or hyaline) on glucose-yeast nitrogen base agar [31], all isolates were fermentation-negative, ureasepositive and utilized citrate [4] and ascorbate but usually not iso-ascorbate as a sole carbon source [3]. Well known as ubiquitous on aerial plant surfaces [2, 13], A. pullulans also has an endobiotic phase [23, 25, 26] that has not been described. Aureobasidin antibiotic production inhibits pathogenic fungi [14, 29]. A dothideaceous loculoascomycete, A. pullulans is either a species complex, or an unusually variable asexual state of Discosphaerina fulvida [32]. In this study I seek the developmental cycle of mycosomes, including their plastid relationship. If plant-derived mycosomes give rise to A. pullulans yeast, then axenic fungal cultures should regenerate mycosomes capable of reversion to A. pullulans. I describe A. pullulans mycosome life history in vitro, and also some in vivo stages. Mycosome distribution was sought in diverse photosynthetic organisms.