A worldwide decline in glass eel seedstock of the commonly cultured eel species has increased demand for the intensive culture of Australian eels, for which knowledge of appropriate feeds and optimum rearing conditions are required. This thesis documents preliminary nutritional requirements of juvenile (0.5 to 1 gram) Australian longfin eels, A. reinhardtii using tank-based experiments. To assess the affects of diet manipulation on growth, feed utilisation and tissue composition under controlled conditions nutritional and morphological experimentation was undertaken at the Freshwater Fisheries and Aquaculture Centre, Queensland, Australia. Dietary protein inclusion levels were found to significantly affect both growth and body composition when dietary protein was increased relative to dietary energy levels. Weight gain and dietary efficiency was greatest in fish fed a diet containing 43 and 48% protein with a significant decrease (P<0.05) in growth at higher protein levels. When dietary energy (E) was decreased excessively in relation to dietary protein (P), whole body protein was unaffected over the experimental period, however, it was insufficient to maintain body energy. When a constant P:E ratio was utilised, total dietary protein and energy were both reduced. This indicates that energy sources in the diet influence the protein requirement offish for optimal growth and the metabolic partitioning of dietary protein between protein synthesis and energy supply. Energy may become limiting with increased dietary protein as protein is metabolised to meet the energy deficit. Growth and feed utilisation were unaffected when plant meals such as soybean meal and cotton seed meal were employed as opposed to fishmeal at optimal protein and energy levels. This suggests that significant economical benefits could be achieved by a combination of protein sources for juvenile longfin eels. Diets in which up to 41.5% of fishmeal protein is replaced by soybean meal were utilised without compromising survival and/or growth performance whilst maximising protein retention and feed efficiency. The influence of different concentrations of linoleic acid (18:2n-6) on growth indicators and tissue composition, and the extent to which the dietary inclusion of flsh oil could be reduced by replacement with sunflower oil in a practical diet for juvenile A. reinhardtii were also assessed. Up to 70% of the total fish oil in diets for juvenile longfin eels was replaced with sunflower oil without adverse affects on either growth or feed efficiency, with linear relationships between dietary sources and tissue fatty acid concentrations observed. Whilst some fatty acids were selectively retained and deposited in the flesh, such that flesh concentrations were always higher than diet concentrations, others were selectively utilised in eel tissue suggesting that these fatty acids are readily oxidised at the concentrations used in this study. There was little affect on the total tissue linolenic (18:3n-3) concentration; however, increased linoleic (18:2n-6) deposition occurred with increasing sunflower oil. An assessment of the effectiveness of mackerel roe to enhance diet acceptability to increase feed intake and the effect of daily feed frequency on growth of slow growing eels found that specific growth rate of slow-growing longfin eels varied little with daily feed number or the addition of feeding attractants. Finally, morphometry revealed that the sex of immature (undifferentiated) individuals may be predicted by total wet weight and relative mouth width. Due to their larger size, the early selection of female eels is commercially desirable. The viability and appropriateness of applying these predictors to undifferentiated animals under commercial conditions has not, however, as yet been assessed.