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We fit asymptotic models to growth data of free-ranging Gila Monsters studied near Tucson, Arizona. We used a mixed-effects modeling procedure that allows for unequal numbers of recaptures by accounting for within-individual covariance and models between-individual variation in growth parameters as random effects. A model selection procedure and diagnostic tests indicated that a von Bertalanffy curve with seasonal cessation of growth and a random effect for asymptotic size fit the data best. Young Gila Monsters in our study grew faster than previously reported growth rates from both field and captive animals, and we discuss potential reasons for this discrepancy.