Dry weight increase of queens and males of the ant Lasius niger after eclosion from pupa was estimated from sequential field samples of eight colonies. Queens increased from ca. 4 mg at eclosion to 15.5 mg at the time of nuptial flight, whereas males remained approximately constant at 0.9 mg (Fig. 1). Queen mesosoma and gaster increased in weight simultaneously up to a total of 13 mg. In queens in nuptial flight condition (≥13 mg) the weights of mesosoma+head and gaster were negatively correlated (Fig. 2). Energy investment in queens and males was estimated calorimetrically. The energy content of queens increased from ca 18.8 J/mg at 4 mg to 27.5 J/mg at 15.5 mg, largely due to the accumulation of fat reserves of 31 J/mg (Fig. 3). The energy content of males was so low (18.2 J/mg) that fat reserves were apparently negligible (Table 1). Oxygen consumption at 20°C was determined for larvae, pupae, and adults of queens and males (Table 2). Data on respiration and energy content were used to estimate the cumulative energy investment by workers per queen and male of Lasius niger from small larvae (<0.3 mg) to flying adults. It was estimated that at least 90 J and 689 J are needed to produce one male and one queen, respectively. Assumptions and sources of error involved in the investment estimations, and the possible effect of a large size dimorphism in ant sexuals are discussed. Special reference is made to an earlier study on dry weight sexual investments in three Lasius niger populations (Table 3). Population means of sexual investments in queens are lower after applying the energetic queen-male cost ratio of this study, but remain in good agreement with theoretical kin selection optima. For the Lasius niger population in the most optimal habitat a better fit to the 3:1 optimum was found than by using dry weight estimation.