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We have measured growth and respiration in black soldier fly (BSF) larvae fed with mixtures of a low-quality substrate (degassed sludge, DS), and a high-quality substrate (chicken feed, CF) in order to elucidate how substrate quality affect larval metabolism and feed conversion into new biomass. The BSF larvae grew faster and became larger the higher the content of CF was in the substrate. Growth followed a sigmoidal curve from where the specific growth rate was determined and compared to measured specific respiration rates, in order to estimate costs of growth, maintenance metabolism, rates of feed assimilation, and net growth efficiency. Specific feed assimilation rates were similar on all substrate mixtures. Maximal specific growth rates were also affected only little unless the larvae were grown in pure DS. In contrast, the cost of growth and the maintenance metabolism were larger the higher the proportion of DS was. High specific growth rates were, in addition, sustained for shorter periods of the time the more DS was included in the substrate mixtures. In effect, higher proportions of the assimilated feed were spent on respiratory purposes instead of being converted into larval biomass and the net growth efficiency decreased the more DS was included in the substrate mixtures. We conclude that substrate quality may affect the conversion of feed into new biomass via alterations of the metabolic performance of BSF larvae and thereby the overall performance of BSF larval cultures.