The paper summarizes results of the study of the occurrence, development and harmfulness of Ernobius mollis (Linnaeus, 1758), which occurred plentifully on Pinus nigra Arn. impaired by drought in the CR in 2000. It is based on laboratory rearings in photoeclectors and numerous analyses of branches (stems) of pines from 5 localities in Brno. Partial studies were carried out in 5 other localities in Moravia and Eastern Bohemia. Under laboratory conditions, imagoes occurred mainly in June–August, viz at a 1 ♂: 3 ♀♀ ratio. Males and females were on average 4.4 and
4.9 mm long, respectively. In living trees, imagoes of substandard dimensions developed. In dead trees, the average length of males and females gradually decreased with increasing damage in the particular years. Immediately after hatching, gonads of imagoes were mostly only partly developed. Females matured sexually during the first hours (maximally during 2 days) of their life in the open. In their ovaries, some 14 to 136 (on average 42.2) eggs occurred. With the increasing size of females their fecundity increased significantly. The development of larvae progressed through 4 instars. One grown up larva destroyed about 1.8 cm² cambium. Wood was damaged to a depth of 2 mm and sporadically to a depth of
15 mm. Imagoes left wood through exit holes of a diameter of 1.1 to 2.1 (on average 1.6) mm in bark. Their development was univoltine. The pest damaged the same wood repeatedly for a period of 5 years.