Multiple mating of females is widespread, and females often obtain direct and/or indirect benefits by mating with multiple males. However, female control of multiple paternity broods is usually only possible in internally fertilising animals with complex reproductive systems including cryptic female choice. Here we present direct evidence that a cooperative polyandrous cichlid fish with external fertilisation, Julidochromis transcriptus, uses environmental factors to manipulate male access to, and therefore paternity of, their egg clutches. Polyandrous females receive more paternal investment from both of large alpha males and small beta males than monogamous females. We used a 'step nest' design, in which large alpha males and small beta males were restricted to the wide and narrow nest areas respectively, and found that when kept in polyandrous trios, females laid their egg clutches exactly at the borderline between the two regions each male could access. In contrast, females in pairs laid their clutches in either the wide or narrow area depending on the size of their mating partner. Our results demonstrate that polyandrous females carefully choose egg deposition site, increasing the direct benefits they receive from mating partners by using environmental structures to manipulate paternity. In this way, externally fertilising J. transcriptus manipulate male behaviour in a manner similar to cryptic female choice in internally fertilising animals. [ABSTRACT FROM AUTHOR]