Class I alpha-mannosidases comprise a homologous and functionally diverse family of glycoside hydrolases. Phylogenetic analysis based on an amino acid sequence alignment of the catalytic domain of class I alpha-mannosidases reveals four well-supported phylogenetic groups within this family. These groups include a number of paralogous members generated by gene duplications that occurred as far back as the initial divergence of the crown-group of eukaryotes. Three of the four phylogenetic groups consist of enzymes that have group-specific biochemical specificity and/or sites of activity. An attempt has been made to uncover the role that natural selection played in the sequence and structural divergence between the phylogenetically and functionally distinct Endoplasmic Reticulum (ER) and Golgi apparatus groups.