To document the extent to which rural and urban-based male traders have sexual contacts away from their place of residence. Fifty-one traders (46 who travel by bicycle and five others) kept daily diaries of all their journeys and sexual contacts for a total of 584 person-weeks. Twenty-five were resident in a trading town where HIV prevalence was about 40% and 26 lived up to 25 km away in rural areas where HIV prevalence was approximately 8%. A total of 2147 return trips were made (mean, 3.7 per week). Eighty per cent were between the trading town and the surrounding rural area. A total of 1377 sexual contacts were recorded (mean, 2.3 per week); 95% of the contacts of urban-based men occurred in the town, 3% in other urban areas and 2% in a rural area. For rural-based men 82% of sexual contacts took place in their home village, 14% in a neighbouring village, 2% in the trading town and 3% in other urban centres. Despite considerable economic interaction there is very little sexual mixing between the town and surrounding rural areas. This may explain why the high HIV prevalence found in some trading towns in Africa has not diffused out to rural areas.