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Filamentous microorganisms populations present in bio®lms from three wastewater treatment plants with a rotating biological contactor system have been studied. Results showed that ®laments are an integral component of the bio®lms, and that they were made up by: Beggiatoa ssp., Haliscomenobacter hydrossis, Sphaerotilus natans, Thiothrix ssp., and the 021N, 0803, 0914, 0961, 1701, 1851, and 1863 Eikelboom's types. Beggiatoa ssp. was the most frequent microorganism followed by S. natans and type 021N. Microthrix parvicella, Gordona amarae (=Nocardia amarae), Nostocoida limicola (I, II, III), and 0041, 0092, 0211, 0411, 0581, 0675, 1702, and 1852 Eikelboom's types appear infrequently.

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The ultrastructure of natural myxospores and glycerol-induced myxospores has been studied in four Archangium gephyra strains (AG3, AG5, AG9, and AG10). The induced myxospores show an oval shape, characteristic of the genus Archangium, and an electron-dense coat surrounds the cell wall. The cytoplasm contains abundant electron-lucent inclusions. Sometimes, a membranous structure appears between the coat and the outer membrane. The fruiting-body myxospores show an oval shape similar to that observed in induced myxospores, but with a slightly thicker outer coat. In many of the natural myxospores, some membranous, lamellar or vesicular structures appear. They are located between the coat and the outer membrane, resembling those that appeared in induced myxospores. These have not been previously mentioned in papers on fruiting-body myxospores. These structures occasionally seem to be fused, giving rise to membranous sacs filled with an amorphous material of medium electron density. We propose that these vesicles are formed from the waved outer membrane and from the fruiting-body matrix. Large, dense, fusiform inclusions with an internal crystalline array are frequently found in the cytoplasm of natural myxospores, and cells during differentiation, in strains AG3, AG9, and AG10. Key words: Archangium gephyra, electron microscopy, myxospores, ultrastructure.

The existing relationships were studied among the different types of fi lamentous microorganisms that appear in the biofi lm of a biological contactor system. Using the hierarchical cluster analysis it was observed that, in all the stages, Beggiatoa sp. and the Eikelboom's types 0803 and 1863 always appeared associated, while Sphaerotilus natans was always associated with the morphological type 021N. The remaining microorganisms were associated in variable forms in the plants. In addition, different association models were obtained according to the season of the year and the stage-season interaction. It has also been observed that a signifi cant correlation exists among the fi lamentous microorganisms we have studied and the different physical-chemical parameters.