Chemical analysis of macroscopic aggregates and whole water samples show that 26–34% of total particulate organic carbon, nitrogen, protein, carbohydrate, lipid, and particulate dry weight in surface waters of the Santa Barbara Channel and 4–8% in the Gulf of California exist as particles of marine snow 3 mm or longer in longest dimension. Twenty percent of these aggregates in the Gulf of California and 5% in the Santa Barbara Channel were of recognizable zooplankton origin. Macroscopic aggregates from the Gulf of California had significantly lower carbon, nitrogen, and protein content and significantly higher ash content than the total particulate matter. Aggregates in both locations had significantly higher carbon:nitrogen ratios than the total particulate organic matter. Calculated sinking rates of macroscopic aggregates indicate that marine snow serves as a major source of vertical flux to the benthos. As enrichment sites for nutritionally valuable substances, macroscopic aggregates are rich food sources and significantly alter the microscale distribution and composition of particulate organic matter in neritic waters of the pelagic zone.