Field‐collected nodules ofComptonia peregrina(L.) Coult. andMyrica galeL. (Myricaceae), infected by the nitrogen‐fixing actinomyceteFrankiasp., were of two types: those that lacked sporangia entirely, designated spore(‐), and those that showed extensive sporangial development, designated spore(+). In spore(+) nodules ofC. peregrina, sporangia began to develop after the differentiation of endophytic vesicles and the concomitant onset of nitrogenase activity. At the onset of sporangial differentiation, infected host cells appeared healthy. However, endophytic vesicles and host cell cytoplasm and nuclei began to senesce rapidly as sporangia developed. Staining of sectioned material with the fluorescent stain Calcofluor White suggested that vesicles, hyphae and young sporangia were enclosed within a host‐derived encapsulation layer, but mature sporangia were no longer encapsulated. In bothC. peregrinaandM. gale, vesicles were more short‐lived in spore(+) than in spore(‐) nodules. Field‐collected spore(+)M. galenodules exhibited a pronounced seasonality of sporangial formation. Sporangia began to differentiate in June, after the formation of vesicles and became more prominent in late summer. Inter‐ and intraspecific cross‐inoculations suggest that the ability to form sporangia in the symbiotic state is controlled by endophytic strain type rather than host genotype or host/endophyte combination. The host may, however, influence the number and seasonal appearance of sporangia formed.