Parasitic infections may cause alterations in host life history, including changes in reproductive investment (absolute amount of energy allocated to reproduction) and reproductive effort (proportion of available energy allocated to reproduction). Such changes in host life history may reflect: 1) a parasite tactic: the parasite adaptively manipulates energy flow within the host so that the host is induced to make a reduction in reproductive effort and reproductive investment, making more energy available to the parasite; 2) no tactic: there is no change in host reproductive effort and reproductive investment simply decreases as a side effect of the parasite depleting host energy stores; 3) a host tactic: the host adaptively increases reproductive effort in the face of infection and loss of body condition, reproductive investment possibly being reduced despite the increased reproductive effort. Females in Alaskan lake populations of threespine sticklebacks (Gasterosteus aculeatus) are capable of clutch production when parasitized by the cestode Schistocephalus solidus despite large relative parasite masses. We analyzed the somatic energy reserves, maturation stage and ovarian mass of female sticklebacks collected from an Alaska lake during a single reproductive season. We found that parasitized females were less likely to carry fully-matured gametes, had smaller ovarian masses, and had lower somatic energy stores than unparasitized females. The relationship between reproductive investment and energy storage did not differ between parasitized and unparasitized females. Thus, reproductive effort did not change in response to parasitic infection. We conclude there was no indication of either a parasite tactic or a host tactic. Simple nutrient theft is involved in the parasite's influence on host reproduction, consistent with an earlier hypothesis that reproductive curtailment in threespine sticklebacks is a side effect.