In recent years environmental researchers have drawn attention to reports on measured contents of drugs in surface water, ground water and drinking water all over the world. Several antibiotics enter aquatic and terrestrial ecosystems through the discharge of effluents from farms. Hydroides elegans is a sedentary fouling tubiculous polychaete available in plenty on the hulls of the ships, fishing boats and floating material on the sea water. Its Fertilization triggers a complex sequence of structural and metabolic transformations in the eggs which results in the initiation of cell division and activation of embryogenesis. Antibiotic were used to assess the effects on various developmental stages of H. elegans in relation to concentration and exposure time. The result of the present study on the effect of flucloxacillin on fertilization and early embryonic stages of H. elegans are presented in four different parameters such as cumulative time, individual stage time, percentage of successful development and EC50 values. The present work confirms that the bioassay using the gametes and embryos of H. elegans can ideally be used to assess the relative toxicity of various drugs.