Spore proteins of different species and isolates of arbuscular mycorrhizal (AM) fungi were compared by PAGE. Reproducibility of protein patterns was assessed by using cultures of the same species either grown on different host plants, or produced during successive propagation cycles and stored up to 5 years. The results consistently showed that host species, different generations and storage, did not affect protein profiles, thus validating the accuracy of the method. Comparison among different geographical isolates of the same species revealed consistent protein patterns. The stability and diversity of spore protein profiles suggested that PAGE could be used to discriminate and identify AM fungal species and isolates. By contrast, the physiological state of spores affected the quality and quantity of bands, with germinating spores showing marked profile changes, as compared to quiescent spores, both in denaturating and native analytical conditions. The disappearance of some polypeptides in germinated spores might be related to the occurrence of storage proteins in AM fungi.