The melanism of Oligia strigilis (L.) occurs in Finland as geugraphical isolates, whereas that of O. latruncula (D. & Sch.) has a continuous distribution. The local populations of O. strigilis differ from each other in the colouration of the melanic moths and in the relationship between the numerical ratio of the colour forms and stage of air pollution. It appears that melanism has arisen independently in several localities, since diffuse gene flow between localities or from outside Finland seems improbable. According to this mosaic distribution and since the oldest melanic moths known from Finland date from 1961, an early stage of industrial melanism might be in evidence. - Two pairings showed that melanic colouration is a dominant character which seems to be controlled by a single gene pair. The role played by multiple allelism and/or incompleteness of the dominance in the variation of the external appearance of the melanics needs further investigation.