Description: Humpback whales in the southern hemisphere migrate in the beginning of the austral winter from their Antarctic feeding grounds to their winter breeding grounds in tropical or sub-tropical waters. Some of these reproduction areas are situated in the waters off the South American coast. However, hardly any information is available on occurrence and distribution of humpback whale in South American waters and especially of their breeding areas in the tropical East Pacific. Therefore a study was conducted in the Machalilla National Park off the coast of Ecuador in the years 1997 to 1999. The goal was to determine whether the humpback whales seen here use the area as a breeding ground, to determine population size and to investigate habitat utilisation, with a special focus on environmental factors which might govern distribution. Also, behaviour patterns and the influence of the newly-developing whale-watching industry were to be a focus of the research. The main methods used were photo-identification as well as observations from the Isla de la Plata, an island about 30km from the mainland. The results of this study showed that the area of the Machalilla National Park forms a breeding and calving area for southern hemisphere humpback whales from early June and to late September. The whales showed typical migration patterns, annual return of individuals, re-sightings of individual whales within a season, presence of young calves and a variety of typical behaviours associated with reproduction, including the forming of competitive groups and the presence of singing humpbacks. The mean distance between breaching humpback whales was found to differ significantly from the distance between non-breaching animals and indicated the use of breaching as a spatial distance holder on the breeding ground. Using results from this study it is hypothesised that the mating system of humpback whales might can be used to explain large scale movements and small-scale habitat use of humpback whales. The best population estimate for the Machalilla National Park using mark-recapture methodology is 405 animals (CJ.: 221-531). Individually-identified animals have been sighted repeatedly within a season as well as from year to year. A total of 93 individual whales were identified using their flukes and the photographs are stored in a catalogue to aJlow comparison with other humpback whale populations in the future. During the year 1997/98 an El Nino event caused a change of relative abundance of humpbacks, but no change in behaviour, group sizes or compositions was noted.