Measurements of muscle dimensions that affect respiration in relation to body weight were carried out in a tilapia,Oreochromis niloticus. The fish used in all measurements weighed 0.65–812.3 g. The data were analysed with respect to body weight using logarithmic transformations (log Y=loga+blogW).The slopes (b) of the log/log regression lines for weight of body trunk red muscle, average cross‐sectional area of muscle fibre, average number of capillaries in direct contact with a muscle fibre, average capillary contact length with a fibre as a fraction of average fibre circumference and number of capillaries mm2of fibre cross‐sectional area [NA(cƒ)] were 1.16, 0.221, 0.084, 0.015, and −0.137 respectively.These results show that there is an increase in muscle cross‐sectional fibre area and number of capillaries in contact with muscle fibres whereas number of capillaries supplying a unit area of muscle fibre decreases during development. There is development of new capillaries with increase in cross‐sectional area of red m