Boron is present in our soil, water and air. Cyanobacteria require it for nitrogen fixation, and vascular plants require it for the formation of cell walls and membranes. I report here how boron affects the growth of embryonic rainbow trout (Oncorhynchus mykiss). Fertilized ovum from the Mt. Whitney rainbow trout strain were incubated at (12.5 degreesC) in Type 1 ASTM ultrapure grade water supplemented with boric acid (99.5% purity) during the 1995 and 1997 spawning seasons. Boron concentrations of the incubation solutions were determined by direct measurement using the curcumin procedure or inductively coupled plasma-mass spectrometry. In the 1995 study boron ranged from 1 to 936 micromol/L. Ca, Na and Mg salts were included in the incubation solutions to approximate concentrations in natural water. In the 1997 study fertilized eggs were incubated in ultrapure water supplemented with boric acid alone over a range from 2.2 to 90.6 micromol/L. The 1995 study used 144 embryos per B concentration and the 1997 study used 96 embryos per B concentration. Growth and teratogenicity were evaluated at the eye, hatch and 2-wk posthatch developmental stages. Boron stimulated growth in a dose-dependent manner in both studies (P < 0.001), and exposure was associated with an increase in B body concentration (P < 0.05). No teratogenic or microbicidal effects were apparent. These results are consistent with those expected of an element essential for vertebrate development. J. Nutr. 2488-2493, 128: 1998