The body length and diameter of muscle fibres from the right hypaxial tail myomeres were measured in 100 sea bass larvae (Dicentrarchus labrax, L.) at ages of 40 to 90 days, from a fish farm in Croatia. The larvae were kept at a temperature of 19-20 degrees C and fed with standard feed for their respective ages. Body lengths of larvae 40-90 days old varied from 8 to 42 mm, increasing significantly in 10-day intervals (p < 0.001), except between days 50 and 60 when there was no increase in the body length. The measured diameters of muscle fibres ranged from 10 to 80 microm, the mean value increasing except between days 50 and 60. The fibre diameter in larvae aged up to 60 days was 10-40 microm, while that in larvae older than 70 days was 51-80 microm. In 90-day-old larvae there were no fibres of 10-20 microm in diameter; however, the number of fibers of 61-80 microm diameter increased. A statistically significant concordance was determined between the body length and fibre diameter of larvae 70 to 90 days old. Between days 50 and 60, arrested growth of the larval body and corresponding decrease in fibre diameter was observed. From day 45 until days 59, larvae were fed with Artemia nauplii and microparticles of formulated diets. From the day 60, minced fish was added to formulated diets. This change in diet probably resulted in the observed growth arrest.