It has been demonstrated by the example of the crucian carp (Carassius carassius) that a 1-hour stay of fish in a combined magnetic field with resonance parameters for calcium ions decreases the proteolytic and amylolytic activities of their intestinal enzymes. It has been found that a 1-hour exposure to a combined magnetic field with resonance parameters for potassium ions has almost no effect on the activity of proteinases, but it decreases the amylolytic activity. It has been noted that the activity of proteases and glycosidases is lower under hypomagnetic conditions. Upon the inversion of the vertical component of the geomagnetic field, the proteolytic activity of the intestinal mucosa in C. carassius decreases, while the amylolytic activity becomes higher compared to the control. Possible effects of magnetic fields on the activity of digestive hydrolases in fish are discussed.