Aggregation behavior of Cordulecerus alopecinus Burmeisteir, a neotropical species of neuropteran from southeastern Brazil is described for the first time. Three roosts were studied during 14 days at the Reserva Florestal de Linhares, Espírito Santo state, Brazil. Males and females of each roost were marked on the first day of observation to evaluate possible flux of individuals among aggregations as well as roost fidelity. Roosts were censused daily at nightfall and at early morning, when notes on the behavior of aggregating individuals were also taken. Four tree species belonging to four families were used as roosting sites. The number of individuals in roosts varied daily, and roost size, as measured by the mean number of individuals, differed strikingly. A relatively low flux of individuals was detected among aggregations and most neuropterans usually returned to their original roost, showing some degree of roost fidelity. The sex ratio of aggregations was strongly biased to females and the observed number of dispersing individuals did not differ by sex. This new data suggests that aggregation behavior is a common feature within the genus Cordulecerus and that aggregations can be more than a random and casual assemblage of individuals, deserving futher quantitative investigations.