The small Rho-type GTPases Cdc42 and Rac1 are key regulatory molecules that control cell polarisation and cell fusion in eukaryotes. However, which functions of these two Rho GTPases are common and which are distinct is still poorly understood. Spore germlings of the filamentous fungus Neurospora crassa are a useful model system in which to analyse this problem because they produce two functionally distinct cellular protrusions – germ tubes and conidial anastomosis tubes (CATs) – that exhibit negative and positive chemotropisms, respectively. On page 1953, Nick Read and colleagues have used N. crassa to identify different functions of CDC-42 and RAC-1 in processes that control cellular polarisation and chemotropism. The authors show that whereas CDC-42 is both necessary and sufficient for normal germ-tube growth, RAC-1 is essential (but not sufficient) for cell fusion and the formation of CATs. RAC-1 and CDC-42 are therefore not functionally interchangeable. The authors next generated Cdc42-Rac-interactive-binding (CRIB)–GFP reporters to differentiate between populations of inactive GDP-bound and active GTP-bound GTPases. Together with custom-designed automated tip-tracking image analysis software, they demonstrate that repositioning of clusters of locally activated GTPases within the tip apices of germ tubes and CATs regulates future growth direction. These data show that CDC-42 and RAC-1 in N. crassa have evolved to control opposite cell chemotropisms resulting in hyphal avoidance and cell–cell fusion, respectively.