In studies on reproductive ecology of pelagic copepods, the role of males is often neglected and male reproductive investment is not well understood. Lately there has been an increasing interest in the topic and it has been indicated that male investment does in fact play an important role in the reproductive dynamics of pelagic copepods. In this experiment, male reproductive investment as a function of food availability has been investigated for the calanoid copepod, Temora longicornis. During mating experiments, male spermatophore production has been quantified after three treatments of decreasing food availability. The results have clearly shown that the production of spermatophores is closely linked to food access. Surprisingly, the production is drastically reduced when food access is limited indicating a large investment into spermatophores. However, the direct costs of the spermatophores seem to be minor when their volume is compared to the male body volume. Traditionally, evolutionary theory assumes sperm to be an unlimited resource, but here it is shown that in environments deprived of food, the spermatophore production is limited.