An experimental aerial survey of dolphins was conducted to investigate the effects of sea state, sun glare, cloud cover, and observer experience on line transect estimates of school density and detectability. Although estimates during rough seas were lower than estimates during calm seas, and estimates from inexperienced observer teams were lower than estimates from experienced teams, these differences were not significant. School density estimates during poor visibility conditions, due to sun glare or cloud cover, were 39% smaller than during good conditions. Therefore, aerial survey designs should position tracklines to minimize glare under and forward of the plane. If possible, sea-state conditions greater than Beaufort 3 should be avoided, and experienced observers should be utilized. J. WILDL. MANAGE. 51(4):836-843 Line transect theory has been used to estimate population densities of cetaceans (Leatherwood 1979, Leatherwood and Show 1980, Scott and Winn 1980, Hammond 1981, Smith 1981, Holt and Powers 1982, Hammond and Laake 1983, Hammond 1984, Cooke 1985, Hiby and Thompson 1985, Holt 1985). The valid use of line transect theory is based upon several requirements (Seber 1973, Burnham et al. 1980) that have been assumed to be true or to have minimal effect on the estimates. One requirement, often suspected of not being met for cetaceans, is that all schools are detected on the trackline during all sighting conditions encountered during the surveys (Scott and Winn 1980, Holt and Powers 1982, Leatherwood 1982, Basson and Butterworth 1984). The ability of observers to detect all schools on the trackline may be affected by sea conditions or poor visibility and by the abilities of the observers. These factors are often confounded with each other or with other variables. For example, surveys that traverse nearshore and offshore tracklines may encounter rougher seas offshore, but animal density may decrease offshore. Thus, sea-state effects may be confounded with actual density. In this paper we present results of an experimental aerial survey for dolphins that investigated the effects of sea state, visibility conditions, and observer performance on detecting dolphins and on density estimates that were determined using line transect methods. We would like to express our appreciation to W. F. Brinkeroff, L. J. Hanson, M. S. Lowry, F. M. Ralston, and W. A. Walker for unselfishly participating in the survey and collecting the data. Special recognition is given to B. J. Vorndam for providing valuable programing assistance during the analyses, and K. S. Raymond and R. M. Allen for completing the illustrations. J. P. Barlow, J. M. Michalski, W. W. Parks, G. T. Sakagawa, S. N. Sexton, and T. D. Smith provided many constructive review comments. We also benefitted from technical reviews by G. Broadhead, K. P. Burnham, D. G. Chapman, W. B. Clark, and P. S. Hammond.