We describe the development of the spinal cord during the post eclosion period of the salmon (Salmo salar). We used a total of 200 newly hatched fry grown in the Aquaculture Research Center of the Universidad de Chile. Environmental conditions were of 90% oxygen saturation. Ambient temperature was maintained at 7 degrees C. At days, 1, 3, 5 and 7, post-hatching, 50 fry were anesthetized and sacrificed by exposure to 5% benzocaine diluted in water, (Kalmagin 20 (R), Farquimica). They were then fixed in 10% buffered formalin and processed by histological technique. For each juvenile a total of 40 serial coronal sections of 5 mu m were taken at the level of the dorsal fin, which were then processed according to cresyl violet techniques. Neuronal quantitation was performed on microscopic images by dissector method. The results obtained were subjected to coefficient Kurtosis test in order to analyze the degree of concentration of values around the central distribution area. The spinal cord of the one-day fry is poorly differentiated. In fry of 3, 5 and 7 days neurons are gradually differentiated, they do not however present the characteristic neuronal distribution inverted "Y" of the adult salmon. The number of neurons increases from 67 +/- 1.7 on day one, to 88 +/- 2.1 on day 7. This observation may be related to the absence of fish swimming movements during days one and three as these fall on the gravel at the bottom of the trays. A determining factor in the acquisition of the morphology of the spinal cord is the start of swimming movements, which occur at around the fifth day post-hatching. Active motor activity allows spinal cord neurons to be recruited and form to activate neural networks, to remain finally in the most efficient circuits. Increasing the number of neurons can be explained by post-hatching neurogenesis as in other teleosts. This study indicates that at the time of hatching, the nervous system is very undifferentiated and that neuron differentiation and neurogenesis occur during the first weeks of life. This knowledge is very important as fish farms take care of eggs, neglecting the nursery stage in the belief that tissues are formed.