AbstractA new and uniquely structured matrix of mammalian phenotypes,MaTrics(MammalianTraits for Comparative Genomics) in a digital form is presented. By focussing on mammalian species for which genome assemblies are available,MaTricsprovides an interface between mammalogy and comparative genomics.MaTricswas developed within a project aimed to find genetic causes of phenotypic traits of mammals usingForward Genomics.This approach requires genomes and comprehensive and recorded information on homologous phenotypes that are coded as discrete categories in a matrix.MaTricsis an evolving online resource providing information on phenotypic traits in numeric code; traits are coded either as absent/present or with several states as multistate. The state record for each species is linked to at least one reference (e.g., literature, photographs, histological sections, CT scans, or museum specimens) and soMaTricscontributes to digitalization of museum collections. Currently,MaTricscovers 147 mammalian species and includes 231 characters related to structure, morphology, physiology, ecology, and ethology and available in a machine actionable NEXUS-format*. FillingMaTricsrevealed substantial knowledge gaps, highlighting the need for phenotyping efforts. Studies based on selected data fromMaTricsand usingForward Genomicsidentified associations between genes and certain phenotypes ranging from lifestyles (e.g., aquatic) to dietary specializations (e.g., herbivory, carnivory). These findings motivate the expansion of phenotyping inMaTricsby filling research gaps and by adding taxa and traits. Only databases likeMaTricswill provide machine actionable information on phenotypic traits, an important limitation to genomics.MaTricsis available within the data repository Morph·D·Base (www.morphdbase.de).