Field-collected 4th-instar Glyptotendipes paripes larvae from a natural lake in central Florida preferred small-sized (< 0.84-1.99-mm-diam) grains of sand to burrow in and make cases in the laboratory. The larvae buried or formed cases within 3 h of introduction into 120-ml paper cups containing 80 ml tap water and 20 g of the small-diameter sand. When placed in sand with grains of different size ranges, 98-100% of the larvae constructed cases in sand < 0.84 mm in diameter, whereas 86% of these larvae built cases in 0.85-1.99-mm-diam sand. The larvae were unable to construct any recognizable cases in coarser sand (2-3-mm diam). The lengths of cases built with smaller sand grains were significantly longer than those constructed in the larger grains used in the experiment. For laboratory colonization and insecticidal bioassay purposes using G. paripes larvae, sand grains < 0.84 mm in diameter should be utilized for better acclimation of the larvae.