Are there two populations of BL Lac objects ?

We present single-epoch infrared photometry of two complete samples of BL Lacertae objects, one radio-selected (RBLs) and one X-ray-selected (XBLs). we find that the RBLs fall into two distinct groups on the (J - H)-( H - K) plane, corresponding to those sources in which the host galaxy makes a significant contribution to the infrared colours and those with higher redshift whose infrared spectra are dominated by power-law non-thermal emission. We also find that the XBLs have colours intermediate between these two groups (although with significant scatter). We show that the infrared colours of Fanaroff-Riley class 1 galaxies (FR1), XBLs and RBLs are consistent with mixing of stellar emission with an increasing proportion of non-thermal emission, and argue that the FR1s, XBLs and RBLs form part of a single population, with the FR1s and XBLs both being at larger angles to the line of sight than are the RBLs, and differing only in the luminosities of their non-thermal components.