Habitat choice of herbivores is expected to be a resolution of a trade-off between food and shelter. The resolution of this trade-off may, however, be dynamic within a species because distinct phenotypes may value these factors differently and the value may vary temporally. We studied this hypothesis in the marine herbivore Idotea balthica (Isopoda), by simultaneously manipulating both food and shelter, and investigated whether the resolution of the trade-off differed between sexes, colour morphs and day and night (i.e. high and low predation risk). Isopods chose between exposing and concealing backgrounds in which the quantity or quality of food varied. When choosing between the backgrounds in the absence of food, females preferred the concealment more than males did. However, in a trade-off situation the isopods traded shelter for food, and females more so than males. Thus, males' lower preference for the shelter was not counterbalanced by a stronger preference for food. The microhabitat use also differed between night and day showing adaptation to diurnally fluctuating predation risk. We suggest that microhabitat utilization of females is more strongly tied to variation in risk and resources than that of males, for whom other factors, such as seeking mates, may be more important.