In this autoradiographic study of protein synthesis during rhizoid differentiation in P vittata, total grain counts increase seven-fold, from 42 grains in the non-differentiated stage to 295 in the premitotic corona stage. This almost doubles to 624 in the postmitotic internally- segmented rhizoid stage (irc) and increases five times to 3,079 in the wheel stage that follows; the count then doubles to 6,120 grains in the later protrusion stage and approximately doubles again to 13,487 in the final young rhizoid stage. These results show that rhizoid initiation, differentia- tion, and growth are attended by significant increases in protein syntheses. Although the grain count over the cytoplasm increases seven-fold in the corona when compared to the non-differ- entiated cell, the relative percentage distribution for this compartment remains unchanged. How- ever, grain counts over the cytoplasm of postmitotic stages increase from 80% (irc) to 96% (wheel) to 97% (protrusion) and to 99% (young rhizoid) of the total count. In contrast, nuclear labeling reached a stationary phase in the irc and in the subsequent postmitotic stages. The first rhizoid in Pteris vittata L. is produced when the spore divides mitotically at germination; however, rhizoid formation may be more clearly observed later in gametophyte development, i.e., in the enlarged, vacuolated thallus cells that mature behind the regions of cell division and enlargement (Crotty, 1967). Increases in cytoplasmic RNA in the vicinity of the spore nucleus during primary rhizoid formation in Matteuccia struthiopteris (L.) Tod. were reported by Gantt and Arnott (1965). Extensive cytochemical studies of Pteris vittata by Crotty (1961, 1967) have provided detailed descriptions and analyses of the events at the light microscope level that accompany secondary rhizoid devel- opment, including RNA, protein, and polysaccharide syntheses. Additionally, these studies described a succession of developmental stages that occurred during the pre- and postmitotic rhizoid differentiation process (Fig. 1). Using a precise autoradiographic technique, Cohen and Crotty (1979) reported that the RNA in the anastomosing cytoplasmic strands around the nucleus of the corona (first stage of rhizoid differentiation) originated in the premitotic nu- cleus of this stage. In a later autoradiographic study, Cohen and Crotty (1990) demonstrated the existence of specific patterns of RNA synthesis in the stages of rhizoid differentiation in P vittata. The purpose of this study was to determine if protein synthesis during rhi- zoid differentiation, as measured by radioactive leucine uptake, could also be quantified and resolved into specific patterns. And, if so, can the results show a meaningful relationship to the earlier studies in P. vittata? In fact, the results of this investigation demonstrate that rhizoid differentiation is attended by