Thirty-day-old fathead minnows, Pimephales promelas, were reared at different pH values in softened Lake Superior water enriched with aluminum: pH 7.5 - 35 micro g al/L, pH 5.5 - 30 micro g Al/L, pH 5.2 - 35 and 60 micro g Al/L, including a background level of 15 micro g Al/L, and at pH 7.5, 6.0, 5.5, and 5.2 at background Al levels. Spawning was greatly reduced at pH 6.0, pH 5.5 - 30 micro g Al/L, and pH 5.5 and failed at pH 5.2 with or without added Al. The adult brood stock exhibited abnormalities at low pH, which could have contributed to poor spawning success or energy utilization: (i) thickened respiratory in the gills, (ii) hyperplasia of primary lamellar epithelium in the gills, (iii) increased number of gill chloride cells, (iv) reduced gill perfusion, (v) immature ovaries and oocyte atresia, (vi) immature and pathologic testes, (vii) abnormal distal tubules and collecting ducts in the kidneys, and (viii) reduced blood osmolality at pH 5.5 and 5.2 when no Al was added. Hatching success and larval survival were reduced when spawning occurred at or below pH 6.0; these larvae often had retarded swim bladder development and yolk absorption and some stages had abnormal gills, kidneys, and liver glycogen reserves. The study further supports the relationship between acidification, histological changes, ionoregulatory disturbances, altered energy metabolism, and recruitment failure.