Aerial surveys covering 5117 linear kilometers were conducted at 91 and 183 m altitude under optimal sighting conditions in an area north of Fyn, Denmark, and in the Bay of Kiel, Germany, in June-July 1991. A total of 116 harbour porpoise, Phocoena phocoena, pods (140 animals) were observed, of which 22 were seen in the Bay of Kiel. Density of pods of porpoises were estimated by use of line-transect theory utilizing measurements of perpendicular distances to sightings and fitting the probability density function by a hazard rate model. The mean of the pod sizes ranged between 1.1 and 1.3, and they were used to estimate the actual number of porpoises present at surface during the survey. The abundance of porpoises in the area north of Fyn was 457 (95% CI: 288–726) which was significantly larger than the estimate of 160 for the Bay ofKiel (95% CI: 96–267). The mean pod size was lower than estimates obtained from shipboard surveys. A more realistic and independent estimate of mean pod size (1.96) was used to calculate abundances of 820 (95% CI: 519–1297) and 251 (95% CI: 153–410) porpoises in the area north of Fyn and the Bay of Kiel, respectively. No corrections were applied for submerged porpoises. Strip census methods are not applicable to harbour porpoise surveys because the width of the strip where an equal detection probability persists is unacceptably narrow. It is recommended that future surveys should be conducted at an altitude of 183 m and that line transect methods should be applied.