A description and illustrations of the mature larva of Deltometopus amoenicornis (Say, 1836) (Coleoptera: Eucnemidae: Macraulacinae: Macraulacini), endemic in the Nearctic region, are given. The description is based on mature larvae collected from pieces of rotten sugar maple wood (Acer saccharum Marshall). Adults were reared from larvae in wood placed in plastic bags and associated with larvae found earlier in the field and preserved. Biological information is provided based on literature records and personal observations. Comparative morphology among larvae of Deltometopus Bonvouloir, 1871 and two other eucnemid genera is briefly discussed. This is the first larval description for Deltometopus.