Abstract: Endocrine disrupting chemicals (EDCs) pose serious risks to the health of animals. As an important and susceptible indicator in examining and assessing ecotoxicological effects of EDCs in aquatic ecosystem, fish behavior is of high ecological significance. Animals are always behaving. Behavioral repertoire represents the consequence of a diversity of physiological and biochemical alterations. It is a link between physiological responses to contaminants and ecological processes for a given species. Behavioral change is one of the most noticeable endpoints produced by EDCs, as toxicants can often disrupt or even eliminate the behavioral patterns that are essential for the fitness and survival of the entire population. However, it has been largely overlooked by ecotoxicologists until the last a few years. Nonylphenol (NP) has been extensively used in industrial and household production and is commonly released from industrial and municipal sewage treatment plants into the aquatic ecosystem. NP is one of the most serious environmental concerned EDCs and is capable of mimicking the biochemical behavior of natural estrogen and interacting with estrogen receptor. Courtship behavior in fish is hormonally controlled, thus it may serve as a sensitive indicator of sublethal exposure to estrogenic compounds. If courtship behavior is suppressed by EDCs, spawning potential would be depressed. Therefore, it is important to evaluate the effects of EDCs on courtship behavior of fishes. However, a standard protocol for assessing the effect of EDCs on courtship behavior in zebrafish (Danio rerio) has not yet been well established. In this study, the effects of exposure to nonylphenol (NP) on courtship behavior and fecundity of zebrafish (Danio rerio) were examined to evaluate the possible hazard of NP to courtship behavior of fish and to pursue a potential biomarker of NP contamination. Fish were randomly divided into six groups. Five groups were exposed to 0.1, 1, 10, 50 and 100 g/L nominal concentrations of NP for 24d, respectively. The sixth (control) group was exposed to acetone only. Acetone concentration in water did not exceed 10 L/L. The contaminants were administered via the water, as 50% of the water was exchanged daily and replaced with water contaminated with NP. Courtship behavior was observed during a 15min period. The frequency of chasing, pushing, tail-nose and quiver was used to quantify the courtship behavior performance of male zebrafish, and the fecundity was studied to assess reproductive success. NP exposure showed remarkably effect on fecundity (P=0.005). A positive correlation (P0.05) was found between the fecundity and the frequency of chasing (R=0.292), pushing (R=0.293), tail-nose (R=0.377) and quiver (R=0.362). No significant differences in the frequency of chasing, pushing, tail-nose, quiver behavior were observed in different NP exposure groups, whereas NP imposed marked influence on the reproductive effort of chasing (P=0.017) and tail-nose behavior (P=0.021). The reproductive effort of chasing and tail-nose behavior in zebrafish might be promising ecologically relevant biomarker or indicator of NP contamination in aquatic ecosystem.