VLBA observations of radio reference frame sources .1.

We present first-epoch simultaneous dual-frequency VLBA 2 GHz and 8 GHz observations of a sample of 42 of the 560 extragalactic sources for which positions were reported by Johnston et al. Many of the observed sources have dominant cores containing the majority of the total flux density and angular sizes less than several milliarcseconds. However, at some level, almost all the observed sources show structure characteristic of a core-jet morphology, often with multiple jet components. Analysis of source structure corrections to the measured VLBI group delay, based on the images presented here, confirm that intrinsic structure contributions can be significant. These observations represent the first in a series of observations intended to image the entire set of sources presented by Johnston et al.