The number of wind-borne female Simulium damnosum Theobald sensu lato migrating across territory devoid of breeding populations (the Onchocerciasis Control Programme, OCP) were monitored daily at widely separated sites downwind. The seasonal curves of numbers recorded each day were remarkably similar in pattern at each site with little blurring (although numbers lessened as distance from source increased). The uniformity suggests that each daily batch of S. damnosum moved downwind en bloc with but a small proportion of laggards. To test this hypothesis, model curves were generated, with different proportions of daily laggards, from the data at each sampling site and compared by chi-squared tests with the observed curve at the next site downwind. It was found that, in general, those models that most closely resembled the downwind curves were those with the smallest proportion of laggards. Other aspects of the migration were also considered, notably the factors affecting loss of numbers as distance increased and the interaction between laggards and loss where it was shown that the greater the proportion of laggards the greater will be the daily loss.