Seed maturation or seed filling is a phase of development that plays a major role in the storage reserve composition of a seed. In many plant seeds photosynthesis plays a major role in this process, although oilseeds, such as castor (Ricinus communis), are capable of accumulating oil without the benefit of photophosphorylation to augment energy demands. To characterize seed filling in castor, a systematic quantitative proteomics study was performed. Two-dimensional gel electrophoresis was used to resolve and quantify Cy-dye-labeled proteins expressed at 2, 3, 4, 5, and 6 weeks after flowering in biological triplicate. Expression profiles for 660 protein spot groups were established, and of these, 522 proteins were confidently identified by liquid chromatography-tandem mass spectrometry by mining against the castor genome. Identified proteins were classified according to function, and the most abundant groups of proteins were involved in protein destination and storage (34%), energy (19%), and metabolism (15%). Carbon assimilatory pathways in castor were compared with previous studies of photosynthetic oilseeds, soybean (Glycine max) and rapeseed (Brassica napus). These comparisons revealed differences in abundance and number of protein isoforms at numerous steps in glycolysis. One such difference was the number of enolase isoforms and their sum abundance; castor had approximately six times as many isoforms as soy and rapeseed. Furthermore, Rubisco was 11-fold less prominent in castor compared to rapeseed. These and other differences suggest some aspects of carbon flow, carbon recapture, as well as ATP and NADPH production in castor differs from photosynthetic oilseeds.Castor (Ricinus communis) is a model heterotrophic oilseed that contains up to 60% fatty acids compared to approximately 20% and 40% in autotrophic oilseeds soybean (Glycine max) and rapeseed (Brassica napus), respectively (Weiss, 2000). Castor oil is a major component of many industrial lubricants and is currently of special interest, as it is being considered for biofuel use (Baldwin and Cossar, 2008;Dyer et al., 2008;Scholz and da Silva, 2008). With its high fatty acid content and recently sequenced genome, castor is a model plant for studying carbon assimilation in nonphotosynthetic oilseeds.