Species are defined by morphology with light and electron microscopy and by molecular phylogeny with, for example, 18S rRNA sequences. When only molecular techniques are used, we define only a phylotype. All techniques need to be used together to create the most complete understanding of the species. This collection of images reflects Guys efforts to document the biodiversity and phylogeny of flagellates, particularly those living in anaerobic and microaerophilic habitats (such as in small ponds) and those living as endocommensal, symbiotic or parasitic of invertebrates (such as termites or snails) and of vertebrates (such as amphibia and rodents). These studies have emphasized electron microscopy to document the features of each genus and group of flagellates and to unravel their phylogenetic relationships, but these studies have also included light microscopy and molecular phylogeny. These studies suggest that protozoan species are more numerous than expected. Indeed most are probably unknown because of large numbers of cryptic species. These organisms provide much for young researchers to discover. Guy has conducted his work at the University of Blaise Pascal in Clermont-Ferrand, France. This image was taken by G. Brugerolle. Image copyright, G. Brugerolle, used under license.