We tested whether the fish Nile tilapia (Oreochromis niloticus) in a monosex male group spends more energy and displays an agonistic profile, differently from males in male-female groups. Such differences are expected because males and females should compete for different reproductive resources. An intruder male (MM) or female (MF) was paired with a resident male and agonistic interaction was quantified during 20 minutes, 10 minutes after pairing and another 10 minutes period 30 minutes later. Energetic cost was evaluated from O2 consumption, determined by Winckler's Method after 40 minutes pairing. Latency for fighting (mean +/- SD, MM = 27.40 +/- 25.15 s; MF = 14.22 +/- 21.19 s; Mann-Whitney test, U = 33.50, P = 0.21) and frequency of the all agonistic acts in the first 10 minutes session (mean +/- SD, MM < 72.30 +/- 25.29; MF < 73.50 +/- 21.65.10/min; Mann-Whitney test, P > 0.10) were not affected by group composition, thus suggesting that each intruder was a potential competitor at the beginning of the agonistic interaction. However, frequency of undulation (a behavior displayed also during courtship) was higher in the MF than in the MM resident fish (mean +/- SD, MM = 3.56 +/- 5.89; MF = 8.56 +/- 4.00.10/min; Mann-Whitney test, U = 15.50, P = 0.01) at the end of the 10 min session. Frequency of flight, however, was lower in MF than in MM intruder (mean +/- SD, MM = 3.90 +/- 4.33; MF = 0.44 +/- 0.96.10/min; Mann-Whitney test, U = 23.50, P = 0.04). Moreover, the agonistic profile in MM groups was composed of more types than in MF groups (less fighting types were exhibited by both resident and intruder fish). Despite the behavioral differences, energy cost in terms of O2 consumption was not affected by group composition (mean +/- SD, MM = 1.93 +/- 0.54; MF = 1.77 +/- 0.46 mgO2 x gDW(-1) x 40/min; Student's t independent test, t = 0.71, P = 0.49).