ABSTRACT: Determining age in crustaceans is inherently imprecise because they molt periodically and do not retain hard structures throughout their lifespan. Morphological measurements are often used to estimate age, but variability in individual growth rate and molt frequency can result in a wide distribution of sizes in a single age class, making size a poor predictor of true age. Concentration of the autofluorescent age pigment, lipofuscin, has been shown to be directly related to actual age in many crustaceans. The present study assessed the potential of using extractable lipofuscin as a method for determining age in the ghost shrimp Neotrypaea californiensis. Following validation of the technique, lipofuscin-based aging was used to determine age structures for 3 populations of N. californiensis, and these were compared to age structures determined using traditional length-based methods. Analysis of lipofuscin revealed up to 13 age classes where the size-based analysis showed only 7. Comparison of mean size-at-age among populations in Oregon and Washington estuaries demonstrated that growth rate varied spatially, probably responding to site-specific environmental factors like food availability and population density. N. californiensis negatively impact oyster aquaculture in Pacific Northwest estuaries. Analysis of extractable lipofuscin proved to be a more accurate method of age determination than body-length measurements and should facilitate more in-depth investigations of basic biological and ecological processes, which will benefit current efforts to develop an integrated pest management plan for N. californiensis.