The phase transition of solutions of the protein filaments F-actin from the isotropic (I) to the nematic (N) liquid crystalline state was studied by quantitative measurements of optical birefringence and fluorescence. The threshold protein concentration for the transition varies inversely with the average filament length, consistent with the prediction of statistical mechanics of rodlike suspensions based on the excluded volume effect. By measurements of local optical birefringence, a range of actin concentration is identified as the transition region between the isotropic and nematic phases. However, local measurements of the protein concentrations detect no discontinuity within a large number of samples in the transition region, suggesting that the I-N transition for F-actin occurs continuously over a defined concentration range. Thus the I-N transition appears to be of a higher order than the 1st, for F-actin of average filament length 3 μm or longer. Additionally, by mixing a tiny number of labeled actin filaments with the unlabeled ones, we observed thermal motions of individual filaments, thus ruling out an alternative picture, viewing an F-actin solution as an amorphous gel in which long filaments are too entangled or even cross-linked to allow partitioning of filaments into domains of discontinuous concentrations. We propose based on these experimental findings that either the extreme polydispersity of F-actin, or the large average filament length itself, renders the I-N transition to be a continuous one, in terms of both the average alignment of filaments and the protein concentration.