Guatemalan Tyrannulet is monotypic. First described as Elainia vilissima Sclater and Salvin 1859. Type locality "Central America". Sclater (1888) reported two specimens from Coban, Guatemala as syntypes, but Warren and Harrison (1971) note that one of these specimens was collected after vilissimus was described, and thus has no type status. Warren and Harrison (1971) mention "several" specimens in the British Museum (Natural History) that they accept as syntypes, at least one of which is from Guatemala.
Occurs in southeastern Mexico (Chiapas), the highlands of Guatemala, El Salvador, and perhaps southern Belize.
The taxonomy of Zimmerius vilissimussensu lato varies among authorities, recognizing anywhere from one to four species. Rheindt et al. (2013) demonstrated that there are at least three non-sister taxa under this name, and suggested several taxonomic adjustments. Their data were less clear on the relationships between nominate vilissimus and Zimmerius parvus (Mistletoe Tyrannulet), sampling just one individual of each taxon and finding that they are in a clade with Zimmerius albigularis(Choco Tyrannulet), and may or may not be sister taxa. Additional sampling is needed, but many authorities recognize vilissimus and parvus taxa as separate species, based on differences in plumage, vocalizations, and habitat.
Jones, A. W. (2018). Guatemalan Tyrannulet (Zimmerius vilissimus), version 2.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.paltyr2.02