Twenty-two competitive swimmers (9 males, 13 females) were compared with 18 controls (11 males, 7 females) for body composition, intakes of selected nutrients, VO2max, hemoglobin, hematocrit and magnesium concentrations in plasma, erythrocytes and whole blood. Subjects ranged in age from 9.5 to 12.9 years. Male swimmers had 6.5% less body fat than did male controls (p less than 0.05) whereas there was no significant difference in body fat between female swimmers and controls. Energy, protein and magnesium intakes for swimmers and controls of the same gender were similar. VO2max was significantly higher in swimmers than in controls for both sexes. Magnesium concentrations in plasma, erythrocytes and whole blood were similar for swimmers and controls. However, concentrations of magnesium expressed as milligrams/deciliter in plasma were 8% higher in male swimmers than in female swimmers (p less than 0.05). There was a significant (p less than 0.05) positive correlation between plasma magnesium and VO2max in males, r = +0.42, but not in females.