In the nematode Caenorhabditis elegans, a well-established model organism for the analysis of nervous system development and function, nerve processes can be labelled in the intact animal with markers based on the Green Fluorescent Protein (GFP). The generation of GFP variants with improved brightness and modified emission spectra potentiated the use of this marker for in vivo labelling of subcellular structures. This made it possible to label different groups of neurons and their axons in the same animal with GFP variants of different spectral characteristics. Here I show with double labelling experiments that spatial relationships of axons in small axon bundles can now be resolved at the light microscopic level. In the future this will largely circumvent the need for time-consuming electron microscopic reconstructions to detect local defects in axon outgrowth. Furthermore, I demonstrate that neuronal processes can now be traced even in the head ganglia, an area of the nervous system that was previously almost inaccessible for analysis due to the compact arrangement of cell bodies and axons.