In studying the possibility of Trojan exoplanets, the size of the stability region around the Lagrangian points L4 and L5 is crucial. We determined the size distribution of the stability region around L4 in the elliptic restricted threebody problem depending on the mass parameter and the eccentricity. The size distribution has a fine structure with several minimum zones.We have shown that these minimum zones are connected with resonances between the frequencies of librational motions around L4. The situation is analogous to the Kirkwood gaps at mean motion resonances in the distribution of the main belt asteroids of the Solar system.