This laboratory study investigated design features for artificial reefs to be used as enhanced juvenile lobster habitat. These features included size of brick shelters, size of rocks in rock piles, and the character of the underlying substrate. Experiments using brick shelters tested the ability of two lobster sizes (50-59 and 82-89 mm CL) to enlarge shelters by digging in sand-gravel bottom. If the gravel size was 1-2 cm both lobster sizes occupied > 90% of shelters by 22 h and the difference between occupancy at 2 and 22 h was not significant. If gravel size was 3-5 cm or 6-8 cm fewer shelters were occupied and the excavation of the sand/gravel took longer. Piles of three sizes of rocks were each placed on hard bottom as well as sand/1-2 cm gravel bottom, and the number of lobsters occupying each rock pile noted after 2-days. Six times more 70-79 mm CL lobsters and 1.7 times more 50-59 mm CL lobsters occupied rock piles on sand/gravel than hard bottom. For all rock sizes and substrates combined, three times more small than large lobsters sheltered in rock piles. The greater sheltering by lobsters on sand/gravel bottom and by smaller lobsters was thought to be related to the size of shelters available or that could be created between the rocks and the bottom by excavating under the rocks. Nearly all lobsters in the piles were located between the rocks and the bottom rather than among the rocks. A final experiment compared occupancy of 0.7 m and 1.2 m diameter rock piles by 60-69 mm CL lobsters where both pile sizes were of large rocks on sand-gravel. The larger piles attracted more lobsters, but neither the number/m² nor number/m circumference were significantly different. In summary, the substratum beneath a rock reef should be considered part of the reef's structure. Reefs housed more lobsters if the reef was placed on substrate they could excavate. Excavating a sand-gravel mix was easier where the gravel was smaller than 3-5 cm. Because lobsters mostly occupy shelters under the reef, reefs several rocks thick would add little to shelter availability. Occupancy of rock piles was related to rock and lobster size.