Bacteriophage significantly influence the nutrient cycles that govern ecological productivity and global microbial diversity. This is achieved by manipulation of bacterial communities through predation via the lytic cycle and/or genetic exchange via phage temperance and lysogeny. Current investigations into phage-host interactions are dependent on extrapolating knowledge from (meta)genomes. However, up to 95% of all phage-derived sequences share no homology to current annotated proteins. This highlights a significant proportion of biological diversity that is essentially unknown. Phage functional biodiversity is of particular interest because it provides details on how phage entities actively influence their environment and host(s). Here I present an alternative method designed to measure functional diversity of unknown phage genes predicted through assembled viral metagenomes. The Multi-phenotype Assay Plates (MAPs) monitor host substrate utilization diversity and subsequent biomass formation of a selected host during expression of an unknown phage gene. MAPs can also be used to capture bacterial functional diversity at the organism and community level. The secondary applications of the MAPs, along with future recommendations for the methodology, are explored providing hypothesis for research on other ill-defined systems.