Microfilaments associated with cytoplasmic streaming in Nitella flexilis internodes can be decorated with heavy meromyosin (HMM) from rabbit, both in vitro in cytoplasmic suspensions, and in situ in glycerinated cell segments. The bound HMM consists of clearly discernible, polarized arrowheads in a regular repeat of 360-380 Å that are similar to those produced on F-actin. In contrast, similar arrowheads or decorations are not evident on P-protein filaments in sieve elements of glycerinated hypocotyl segments of Phaseolus vulgaris L. treated with HMM. Thus, these results contradict a recent claim that P-protein binds HMM and is actin-like. The mass of other evidence now available from diverse studies indicating that P-protein does not consist of actin or tubulin is discussed.