Malaise traps have become one of the chief tools of systematists and others involved with surveying and inventorying the diversity of insects. Limited evidence for greater overall diversity (richness and abundance) in catches of traps with fine mesh has been proposed and greater diversity of large bodied insects in traps with coarse mesh. As part of an extensive inventory of the insects of the Grand Staircase – Escalante National Monument in Utah we present evidence regarding diversity biases and body size biases from a replicated experiment comparing catches from traps of the two mesh sizes. Trap arrays were placed in two adjacent habitat types, open shrub steppe and dry coniferous forest to further test potentials for habitat-based mesh biases. In one analysis we will present Order level diversity and size information. Our more detailed analysis will concern Diptera at the family level. Particular attention was paid to the Diptera because of our personal expertise with identification, because they have been clearly identified as the chief component of malaise trap samples elsewhere, and because they span a wide range of body sizes of trappable insects.