A spaghetti tag for marking juvenile spiny lobsters, Panulirus argus, was evaluated for retention by the lobsters in Biscayne Bay, Florida. Floy FD-68B tags were inserted into the dorsolateral extensor muscle between the cephalothorax and the abdomen. Mean monthly tag loss was 3.8% for 9 months, or about 10% per molt for the first three molts after tagging. These tags effectively marked P. argus as small as 35-mm carapace length for up to 27 months, but for studies of population dynamics they were only useful for about 12 months.