Periaxoplasmic ribosomal plaques (PARPs) are systematically recurring ribosome-containing structural domains located in the F-actin-rich periphery of axoplasm in myelinated fibers. In contrast, endoaxoplasmic ribosomal plaques (EARPs) are small, oval-shaped ribosomal aggregate structures randomly dispersed within the axoplasm of unmyelinated squid giant axons. Ribosomes are attached to a superficial plaque-like structural matrix, which "caps" the domain at the outer cortical margin and appears fragmented in subcortical axoplasm. As such, the matrix represents a novel hallmark of PARP domains. Molecular markers concentrated in PARP domains include beta-actin mRNA, ZBP-1, SRP54, myosin Va and kinesin II molecular motor proteins. Rapid axoplasmic transport of microinjected heterologous radiolabeled BC1 RNA to putative PARP domains, mediated pari passu by microtubule- and F-actin-dependent systems, suggests that translation machinery, anchored by the matrix could provide targeted destinations for RNA trafficking. As distributed local centers of protein synthesis along axons, PARPs are likely to share modes of expression in common with other translational subdomains in neurons.