Phosphorus (P) deficiency is a major problem in northern Australian cattle but the response of bone tissue to P deficiency has not been well examined in cattle. Three separate experiments investigated the effect of low or adequate P diets in growing steers (n=30), in young cows during their first pregnancy and lactation (n=40) or in older pregnant cows following a lactation (n=40). All diets were adequate for calcium. Cattle were fed treatment diets in individual pens and serial measurements of nutrient intake and liveweight were performed. Plasma samples were collected for P concentration and bone biomarker and hormone analysis. Left and right side surgical rib and tuber coxae (ilium) biopsies were obtained at the start and end of treatment periods (6-12 weeks apart) and routine histomorphometry analysis was performed. In all experiments low dietary P led to biopsy findings of osteomalacia (thick osteoid seams, significantly increased osteoid volume, poorly defined tetracycline labelling, reduced bone mineral apposition rate) as well as osteoporosis (significantly reduced rib cortical thickness and trabecular volume and thickness, reduced bone formation rate and increased osteoclast surface). Low P diets had the greatest effect on bone during lactation and rapid growth. P deficient pregnant cows were able to deposit bone after weaning a calf but this was much less than in cows on adequate dietary P. Compared to cattle fed adequate P, those fed a diet low in P had very low plasma inorganic P, slightly increased calcium and increased plasma Carboxy-terminal telopeptides of Type I collagen and reduced plasma osteocalcin indicating increased bone resorption. P deficient cattle had very low plasma parathyroid hormone (PTH) but increased plasma 1,25-dihydroxy Vitamin D3 and bone alkaline phosphatase (BAP). Serial bone biopsy was useful in identifying changes in cortical and trabecular bone structure and mineralisation in P deficient cattle.