Common bed bugs, Cimex lectularius L., usually occur in aggregations. We studied the effects of sex-ratio on dispersal and aggregation behavior of adult bed bug populations. Ten blood-fed adult bed bugs were placed into Petri dishes at sex-ratios representing all-male, male-biased, normal (50% male and 50% female), female-biased, or all-female populations. Number of aggregations and loner bed bugs, as well as the sex of each bed bug, were recorded over one week. In the normal, female-biased, and all-female populations, 65-68% of bed bugs aggregated. This was significantly higher than male-biased and all-male populations, which had 56-58% aggregated. Percentage of loner bed bugs significantly differed according to sex. All-female and female-biased populations had 34-38% of females as loners while the normal and male-biased populations had 51-61% female loners. All-male populations had 44% of males as loners while the normal and female-biased populations had only 9-18% male loners. Loner bed bugs could be adult females dispersing to avoid multiple traumatic inseminations, or males dispersing to find a mate. Because bed bug control treatments concentrate on areas where aggregations occur, these lone, dispersed bed bugs may be a major cause of treatment failure.