A microplate-microtubule array was observed in Anabaena sp. (B-378). This structure consists of an arched plate, about 8 nm thick, and various microtubules, 12 nm in diameter and 50 nm long, arranged in rows. The microtubules project at right angles from one side of the plate into the cytoplasm or towards the plasma membrane. Up to twelve microplate-microtubule arrays were observed in a single section of a cell. Microfilaments, about 2.8 nm in diameter and of undetermined length, were observed in four isolates of Anabaena. The microfilaments were always found in bundles, nwhich varied in size, up to 0.63 mum across and 0.91 mum long. Microtubules, 10 nm in diameter and about 150 nm in length, were observed associated with one facet of polyhedral bodies in 8 out of 20 isolates of Anabaena. The microtubules occurred in groups of up to 20 or more, and were always oriented with the long axis parallel to the facet of a polyhedral body. In cross section, the microtubules had an electron transparent lumen 5 nm wide and a wall 2.5 nm thick. These structures are compared to previously described microtubules and microfilaments.