A study of the in vitro viability of VA mycorrhizal propagules [Glomus caledonium (Nicol. & Gerd.), Gigaspora margarita (Becher & Hall)], indicated that none could be used for continuous in vitro cultivation. Large numbers of nuclei (at least 20000 for G. margarita) were found in the chlamydospores. Nuclear division could not be demonstrated in vitro, during or after spore germination using various cell cycle inhibitors, or by direct isotope labelling of nuclear DNA. Manipulation of secondary messengers (cAMP, calcium) and the microtubule or micro-filament system did not increase the in vitro growth of the tested VA mycorrhizal strains.