The actin-related protein Arp1 (or centractin, actin RPV) is the major subunit of dynactin, a key component of the cytoplasmic dynein motor machinery   . Of the ubiquitously expressed members of the Arp superfamily, Arp1 is most similar to conventional actin    and, on the basis of conserved sequence features, is predicted to bind ATP and possibly polymerize. In vivo, all cytosolic Arp1 sediments at 20S  suggesting that it assembles into oligomers, most likely dynactin - a multiprotein complex known to contain eight or nine Arp1 monomers in a 37 nm filament . The uniform length of Arp1 polymers suggests a novel assembly mechanism that may be governed by a 'ruler' activity. In dynactin, the Arp1 filament is bounded by actin-capping protein at one end and a heterotetrameric protein complex containing the p62 subunit (D.M. Eckley, S.R. Gill, J.B.B., J.E. Heuser, T.A.S., unpublished observations) at the other . In the present study, we analyzed the behavior of highly purified, native Arp1. Arp1 was found to polymerize rapidly into short filaments that were similar, but not identical, in length to those in dynactin. With time, these filaments appeared to anneal to form longer assemblies but never attained the length of conventional actin filaments.