Although individual electronic tagging using PIT (Passive Integrated Transponder) tags is well established, it is mainly used for fish > 60 mm in length. Since electronic tagging is an ideal identification method, we used RFID (Radio Frequency Identification) microtags (6 mm in length, 1 mm in diameter, 10 mg in mass) to characterize individual fish from the early stages of their development and throughout their lifecycles. We used sea bass, Dicentrarchus labrax, (105 day-old fish weighing between 100 and 1100 mg) and studied the effects of intra-coelomic tagging in half the population using different endpoints including survival and tag reading, growth over 6 months and swimming responses. Dead fish were counted daily, biometric data were collected at 21 to 28 days intervals and fish were sexed at the end of the experiment. Behavioral swimming responses following a sudden dark challenge were evaluated after the first three biometric measurements (immediately after tagging, and then 21 and 42 days later). After 6 months, mean survival was 69 %, the smallest surviving fish weighed 197 mg at tagging, and success in tag reading was 79 % for the size class 300–400 mg after 63 days. No negative effects were observed on growth and most deaths occurred within 15 days after tagging with a first peak after 5 days but that did not particularly affect the smallest individuals. Differences in swimming responses were detected between tagged fish and untagged controls immediately after the surgical procedure with tagged fish swimming more than controls for 45 minutes. However, after 21 days, the tagged fish swam less than the untagged controls after being placed in the observation tank for 1 h45. At 42 days post-tagging, no further differences were observed. Overall, results suggested that the tagging method was suitable for fish as small as 400 mg and 36 mm in total length. This type of early tagging method is invaluable for the longitudinal monitoring of individual biological traits (e.g. growth) or for repeated assays with the same individual at distinct time points (e.g. behavior studies). We used it for a first evaluation of early growth differentiation between sexes and demonstrated a 31% mass gain difference in females that was visible as soon as the fish reached 105 days old. This confirms the potential of the method for the selection of early-expressed character traits which could lead to rearing cost savings for the aquaculture research field.