Black worms in your bathtub are likely drain fly larvae that breed in the organic buildup inside drain pipes. Clean the drain thoroughly using a stiff brush to remove biofilm and sludge. Pour enzyme drain cleaner to dissolve organic matter where larvae feed. Baking soda and vinegar followed by boiling water also help. Fix any leaks creating moisture. Keep drains clean monthly to prevent recurrence. These larvae are harmless but indicate dirty drain conditions.