Symbiosis is the association between two non-related organisms. The common yellow-banded millipede, Anadenobolus monilicornis (Diplopoda: Spirobolida: Rhinocricidae) and the protist Enterobryus luteovirgatus sp. nov. (Ichthyosporea: Eccrinales), a species of hair-like microorganism that inhabits its gut, form a commensalistic relationship. The genus Enterobryus was once part of a fungal class (Trichomycetes), but now it is classified as a protist. Other Enterobryus species have been reported associated with non-carnivorous arthropod hosts including beetles, crabs and millipedes. Yellow-banded millipedes from Guanica Dry Forest, Puerto Rico were collected to study the prevalence of Enterobryus species. A new Enterobryus species that inhabits A. monilicornis is herein described. Traditionally, Enterobryus species are difficult to identify due to high intraspecific variation. Thus, statistical analysis of character measurements is included in an attempt to investigate character stability. Millipedes were dissected; gut linings with attached Enterobryus were placed on slides and preserved. Morphometric data of thalli, sporangiospores and holdfasts presented a normal distribution of parameters except for the basal disk width of the holdfast, which showed extreme variation. This character, although used to described Enterobryus species is not reliable in E. luteovirgatus when using the mean or range values in taxon descriptions.