Copepods are said to be the key link between the primary producers and higher trophic levels. They inhabit all oceans and are found in almost all marine communities. In the ocean, it is common to observe skewed sex-ratios of copepods which fluctuates throughout the year. Still, how or if the sex-ratio is influencing male reproductive investment has yet to be investigated. The main objective of this study was to determine the influence of sex-ratio on the reproductive investment of the male copepod Temora longicornis. The study was conducted in May 2012 at Gulmarfjorden in west Sweden. T. longicornis was incubated for four days on a rotating wheel in three different treatments: male-skewed, female-skewed and gender balanced. The results indicated no adjustment of spermatophore production in any of the three treatments. However, a relationship between male body size and spermatophore production rates was observed. Spermatophore size and production rate were negatively correlated in the male-skewed treatment and positively correlated in female-skewed and gender balanced treatment. The conclusion is that sex-ratio has no influence on the spermatophore production. However, adjustments in reproductive investment cannot be excluded on the basis of the spermatophore production only.