This longitudinal study builds on existing research exploring the developmental course of infants’ negative reactivity to frustration in a sample of 84 irritable infants. We investigated whether infants’ negative reactivity to frustration differed during the first year as a function of infant attachment classification. Various elements of the designs of previous studies investigating negative reactivity and attachment preclude the strong conclusion that negative reactivity develops differently as a function of attachment. Thus, we utilized the same observational assessment of infant negative reactivity, conducted without parental involvement, at 5 and 12 months. One proposition, based in attachment theory (Bowlby, 1969, Cassidy, 1994), is that relative to secure infants, insecure-avoidant infants come to minimize their negative emotional reactions, whereas insecure-ambivalent infants come to maximize their negative emotional reactions. As expected, we found that at 5 months, attachment groups did not differ in reactivity, but at 12 months, insecure-avoidant infants were the least reactive, followed by secure infants, and insecure-ambivalent infants were the most reactive. Results are discussed in terms of conceptualizing the development of emotion regulation and their implications for future