The barnacle Balanus amphitrite (=Amphibalanus amphitrite) is a major marine biofouling invertebrate worldwide. It has a complex life cycle during which the larva (called a nauplius) molts six times before transforming into the cyprid stage. The cyprid stage in B. amphitrite is the critical stage for the larval decision to attach and metamorphose. In this study, proteome and phosphoproteome alterations during cyprid development/aging and upon treatment with the antifouling agent butenolide were examined with a two-dimensional electrophoresis (2-DE) multiplexed fluorescent staining approach. Optimized protein separation strategies, including solution-phase isoelectric fractionation and narrow-pH-range 2-DE, were used in a proteomic analysis. Our results show that the differential regulation of the target proteins is highly dynamic on the levels of both protein expression and posttranslational modification. Two groups of proteins, stress-associated and energy metabolism-related proteins, are differentially expressed during cyprid development. Comparison of the control and treatment groups suggests that butenolide exerts its effects by sustaining the expression levels of these proteins. Altogether, our data suggest that proteins involved in stress regulation and energy metabolism play crucial roles in regulating larval attachment and metamorphosis of B. amphitrite. © 2010 American Chemical Society.