Biomass-specific, dark respiration rates of microalgae vary by almost two orders of magnitude from 0.01 to 0.6 d−1. In general, dark respiration rates increase with growth rates in both intraspecitic and interspecific comparisons, but the precise relationship between respiration and growth rate varies. Under optimal conditions, respiration rates are about 20-30% of growth rates, but the ratio of respiration to growth increases under suboptimal conditions. The intercept of plots of respiration rate vs. growth rate varied from < 0.01 to 0.4 d1, and the slopes of these relationships varied from < 0.1 to 1.0. Minimum maintenance metabolic rates of about 0.01 d 1 have been estimated for some microalgae by extrapolation of the initial slope of the light curve for growth to a hypothetical negative growth rate in darkness. The dimensionless total energy cost of synthesis and maintenance for heterotrophie growth of microalgae using either endogenously or exogenously supplied carbohydrates is typically 0.4-1.2 for growth with ammonia or urea as a nitrogen source and 0.7-1.4 for growth with nitrate as the nitrogen source.