ABSTRACT: Behavioral differences between males and females can lead to sex-biased dispersal (SBD), and tube-building amphipods are good model organisms to study this process. Depending on the species, males or females have a different affinity to their tubes, and one sex may be more mobile than the other. This distinct dispersal behavior and the scales at which it occurs are not fully understood. Here, we tested the SBD hypothesis at different spatial scales in Cymadusa filosa using direct and indirect approaches. We conducted laboratory (small scale) and field experiments (local scale), and molecular analyses (local and large scale). Laboratory experiments indicated male-biased dispersal over small scales (cm), whereas over local scales (m), field experiments showed similar colonization rates of previously cleaned fronds for both males and females, suggesting no SBD. A higher proportion of juveniles than expected had colonized these fronds, suggesting that juveniles have higher dispersal rates than adults. Also, neighboring individuals were not more genetically related than spatially distant individuals. Over large scales (km), molecular analyses did not indicate SBD, and there were no differences in genetic structure between sexes. Our results showed that SBD depends on the spatial scale. Combining different approaches, we showed that the dispersal of males and females over local and large scales is sufficient to cause a lack of genetic differentiation within each sex, despite small-scale SBD.