Mycetosporidium jacksonae n.sp. occurs as a parasite in the intestinal epithelium and the malpighian tubules of weevils of the genus Sitona. The life history comprises vacuolated and compact plasmodia in which arise ovoid or spherical multinucleate bodies resembling coccidian schizonts. Sporulation results in the formation, within thin-walled sporangia, of 8-nucleate, biconvex spores with densely staining, resistant walls. Other types of multiplication occur in the form of small uninucleate, fusiform bodies developed directly from the plasmodia; and of small uninucleate, ovoid or fusiform bodies, four to six of which are formed within each chamber of a multilocular sporangium.

The parasite resembles M. talpa Léger & Hesse 1905, but differs in some characters and, consequently, the new species M. jacksonae is made for it.

The genus Mycetosporidium is of uncertain systematic position and appears to be related to the Mycetozoa, but it also resembles the Haplosporidia in some characters.

(The slides upon which the description of M. jacksonae is based are deposited in the Molteno Institute of Biology and Parasitology, University of Cambridge.)