Carbon and nitrogen isotopes have been widely used in the investigation of trophic relations, energy pathways, trophic levels and migrations, under the assumption that δ 13 C is independent of body size and that variation in δ 15 N occurs exclusively due to ontogenetic changes in diet and not body size increase per se. However, several studies have shown that these assumptions are uncertain. Data from food-webs containing an important number of species lack theoretical support on these assumptions because very few species have been tested for δ 13 C and δ 15 N variation in captivity. However, if sampling comprises a wide range of body sizes from various species, the variation of δ 13 C and δ 15 N with body size can be investigated. While correlation between body size and δ 13 C and δ 15 N can be due to ontogenetic diet shifts, stability in such values throughout the size spectrum can be considered an indication that δ 13 C and δ 15 N in muscle tissues of such species is independent of body size within that size range, and thus the basic assumptions can be applied in the interpretation of such food webs. The present study investigated the variation in muscle δ 13 C and δ 15 N with body size and body mass of coastal fishes and cephalopods. It was concluded that muscle δ 13 C and δ 15 N did not vary with body size or mass for all bony fishes with only one exception, the dragonet Callionymus lyra. Muscle δ 13 C and δ 15 N also did not vary with body size or mass in cartilaginous fishes and cephalopods, meaning that body size/mass per se have no effect on δ 13 C or δ 15 N, for most species analysed and within the size ranges sampled. The assumption that δ 13 C is independent of body size and that variation in δ 15 N is not affected by body size increase per se was upheld for most organisms and can be applied to the coastal food web studied taking into account that C. lyra is an exception.