Grapsid crabs in the subfamily Sesarminae are key faunal components of many intertidal mangrove ecosystems. Although their feeding and burrowing activities play important roles in the processing of plant material and nutrient cycling, relatively little is known about ecological interactions that regulate populations or influence species composition of intertidal crab assemblages. We used manipulative microcosm experiments to explore the potential effects of both intra- and interspecific adult–juvenile interactions in populations of Sesarma (Perisesarma) bidens (de Haan 1835) and S. (Holometopus) dehaani H. Milne Edwards 1853, two species of crabs common in mangrove habitats of Hong Kong. Survival of juveniles (3–10 mm carapace width, CW) held alone and exposed to either a conspecific or heterospecific adult crab (14–30 mm CW) was measured in microcosms (ca. 2.8 l) containing both physical structure and alternative food (mangrove litter). In the absence of adults, juvenile survival of both species after 3 days was 95–100% at densities of one, two and four juveniles per microcosm. Adults functioned as cannibals and intraguild predators by consuming both conspecific and heterospecific juveniles, but the proportion of juveniles consumed was not density dependent (i.e. predatory adults did not exhibit a functional response). There was no difference in the effect of male and female crabs on juvenile survival, but there was an asymmetrical interspecific effect. Fewer H. dehaani juveniles survived in the presence of heterospecific (52%) than conspecific (75%) adults, while survival of P. bidens juveniles did not differ between adult species (85% with heterospecific adults and 78% with conspecific adults). If cannibalism is important in population regulation of these semi-terrestrial crabs, our findings suggest that the mechanism is unlikely to involve a strong functional response of adults to changes in juvenile density. However, juvenile survival in the intertidal environment may be regulated by changes in adult density and availability of refugia. Based on the observed asymmetrical interspecific effect of adult species on juveniles, we predict that P. bidens should tend to reduce H. dehaani populations in areas where the two species overlap through a mechanism of stage-structured intraguild predation.