1. To evaluate differences in energy allocation to gonadal development of resident and anadromous Brown Trout, Salmo trutta, we analysed tissue composition and energy contents of adults of the two morphs from three small streams in southernmost Norway. Somatic energy per unit mass of anadromous spawners (5·0 ± 0·6 kJ g–1 wet weight) was 1·1 times that of the residents. This difference was chiefly due to a 1·4 -times higher lipid content in anadromous (4·9 ± 1·5%) than resident trout. The somatic protein and carbohydrate contents per unit mass were higher in resident than anadromous fish.
2. Somatic energy content per unit mass increased linearly with body length in resident but not anadromous fish. This increase was due to a significant increase in lipid content with fish size in resident contrary to anadromous trout.
3. The mass-specific energy content of the ovaries was higher for residents (8·4 ± 0·4 kJ g–1 wet weight) than anadromous (7·81 ± 0·5 kJ g–1 wet weight) trout. This was due to higher lipid and protein contents per unit mass in resident than anadromous females. In males, no similar differences between morphs were found (5·9 ± 0·8 kJ g–1 wet weight). The mass-specific energy content of the ovaries was c. 1·4 higher than that of the testes, mainly caused by a 2·3–2·6 higher lipid content per unit mass of eggs.
4. The mean gonadosomatic index (kJ %) was higher in anadromous (34·0 ± 9·4) than resident (28·6 ± 5·5) females, but not in males (≈ 3%). In females, but not in males, gonadal energy increased linearly with increasing somatic energy of the fish.
5. Female gonadal energy at corresponding somatic energies was higher than that of males. On average, the energetic investment in gonads relative to soma of first-time spawners in the year of maturation was estimated at 11% in males and 67% in females.