Synchrotron x-ray diffraction was used to identify the oligomers that formed during the earliest stages of actin polymerization. Solution diffraction patterns from G-actin (monomer) and from F-actin (polymer) contain information about the size and shape of actin monomers and the length, width, and subunit organization of filaments. Comparison of patterns collected during polymerization reveals an increase in scatter at spacings greater than 9.0 nm; formation of scattering bands at 5.4,4.9, and 3.4 nm; formation of a scattering minimum at 6.5 nm; and the presence of an isosbestic point at 9.0 nm. These scattering bands arise from the formation of, and organization of subunits in, filaments. At various actin concentrations (0.37-5 mg/ml), the change in scatter in these regions follows simple exponential kinetics with no detectable lag. Our analysis of the x-ray patterns shows that by 0.4 sec after mixing, most of the actin has formed dimers, which then rapidly incorporate into oligomers. At 4 mg/ml the early oligomers increase in length to greater than 30.0 nm within 10 sec. These results suggest that under our conditions actin molecules condense into filaments without the rate-limiting formation of nuclei.