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tokens: ellipticals, reported in Fig. 3 , also occurs within the uncertainties considering five different redshift intervals for the ellipticals, from z = 0.1 to z = 0.35, each containing ∼200 sources. This is apparent in Fig. 3 , where the cases of ellipticals at 0.1 z 0.15 and at 0.1 z 0.33 are reported. Figure 3. Average excess surface density of galaxies (detected in i band, with magnitude threshold M * + 2) as a function of the projected distance from the BL Lac objects (green filled circles), inactive ellipticals matched in redshift with the BLL sample (orange filled triangles), FR Is (violet open stars) and inactive ellipticals (grey open triangles) matched in redshift with the FR I sample. The errors are the standard errors of the mean. The sample of FR I radio galaxies is limited to 0.1 z 0.15 (see Section 2 ), therefore, we can only do a direct comparison with the sample of inactive ellipticals in the same redshift range. We found that within ∼0.5 Mpc the excess density of the FR I radio galaxies is a factor of 2 larger than that of ellipticals (see Fig. 3 ). Since we have shown that the galaxy environment of ellipticals does not change in the redshift range 0.1 z 0.35, we are confident that also the comparison of environment between BLL and FR I radio galaxies is reliable. The result is illustrated in Fig. 3 . In particular, within 0.5 Mpc the excess density is ∼13 Mpc −2 for the FR Is, while for the BLLs it is ∼6 Mpc −2 . 5 CONCLUSIONS We find that the galaxy environment of low-redshift BL Lac objects is indistinguishable from that of low-redshift inactive ellipticals of comparable galaxy luminosity. This result is similar to that found by Karhunen et al. ( 2014 ) and Sandrinelli et al. ( 2018 ) who considered large samples of low-redshift quasi-stellar objects (QSOs), finding an overdensity within 0.5 Mpc of ∼1.1. So it seems that in the local Universe BLLs, quasars and inactive ellipticals are similar in terms of surrounding galaxies. We also find that the clustering of galaxies around radio galaxies of FR I type is on average a factor of ∼2 richer than that of BL Lac objects. In spite of the difference in the redshift distribution this difference appears robust since no variation of galaxy environment is found for inactive elliptical galaxies of same luminosity up to z ∼ 0.3 (see details in Section 4 ). This result is further illustrated in Fig. 4 , where we report the cumulative distributions of the excess density for the two classes of objects within the projected distance 350 kpc. The medians are 5.7 and 13.6 Mpc −2 for BLLs and FR Is, respectively, and the Kolmogorov–Smirnov (KS) test yields a p -value 4 × 10 −4 . The population of BLLs is positioned at lower excess densities with respect FR Is. Figure 4. The comparison of the cumulative distributions of excess galaxy density (within 350 kpc) of BL Lac objects (0.1 z 0.33) and FR I radio galaxies (0.1 z 0.15). The two distribution are significantly different (see text). The