The aim of this study was to develop two behavioral tests (swimming velocity and avoidance behavior) specific for the common prawn, Palaemon serratus, and to investigate the effects of sublethal concentrations of fenitrothion on behavior and on several biomarkers. In a first bioassay, mortality was investigated in prawns exposed during 96h to concentrations of fenitrothion ranging from 39 to 40000ngL(-1). The effects on swimming velocity and biomarkers were determined in prawns exposed to the sub-lethal concentrations of fenitrothion (from 39 to 625ngL(-1)). A specific newly bioassay was developed to assess the capability of prawns to avoid the toxicant (avoidance test). Finally, in order to determine the effects on biomarkers during the avoidance test, prawns were collected at different times of exposure (30, 60, 90 and 120min). Results showed that prawns exposed to the sub-lethal concentrations of fenitrothion exhibited a significant inhibition of swimming velocity with a LOEC of 313ngL(-1). A significant inhibition of both eye AChE (LOEC=78ngL(-1)) and muscle ChE (LOEC=156ngL(-1)) activities were also observed. Results from the avoidance test indicated that animals significantly avoided fenitrothion (78ngL(-1)). However, this capability was not observed in prawns exposed to 156ngL(-1) fenitrothion. Prawns exposed to fenitrothion showed alterations in enzymes involved in the production of energy (LDH and IDH) possibly in an attempt to cope with additional energetic demands. Impairment of locomotion and avoidance may lead to alterations at the population level. Thus, the present behavioral tests can be useful as ecologically relevant tools for ecological risk assessment.