> Abstract Growth rates of marine bacterial isolates on particulate organic substrates were measured using a novel apparatus which restricts bacterial cells to the uptake of hydrolysate produced from particulate substrates only by enzymes that are actively released from the bacterium into the culture medium. Significant, varying growth rates were measured for four different marine bacteria, using three different, ecologically significant particulate organic substrates (preparations of amylopectin, chitin, and animal hide). Growth rates sometimes approached but were usually lower than rates that have been reported in laboratory experiments using dissolved organic growth substrates. These results are consistent with recent model predictions and have important implications for microbial ecology and material cycling in diverse liquid-bathed environments.