Specific gravity and diameter affect the potential and rate of downstream transport of semi-buoyant fish eggs. Salinity and suspended sediment were found to significantly alter both of these physical properties of the egg of the endangered Rio Grande silvery minnow (Hybognathus amarus). Under control conditions (no salts added to elevate salinity) egg specific gravity for H. amarus declined significantly in the first hour after fertilization as the perivitelline space of the egg filled with water. Egg specific gravity achieved a minimum value by 12 hours post-fertilization, which was approximately constant until hatching approximately 48 hours post-fertilization. Egg specific gravity was highly dynamic and it depended on the salinity of the water surrounding the egg. An egg transferred to water with a different salinity initiated immediate change in its specific gravity. Egg diameter and specific gravity of H. amarus eggs were reduced strongly by salinity but only modestly so by suspended sediment. In contrast, the length of newly hatched larvae was not affected by either salinity or suspended sediment. Assuming eggs were spherical, the reduction in egg diameter caused by increased salinity decreased egg volume by more than 20% and egg surface area by more than 15%. Our experiments showed that artificial eggs (Gellan beads) were a close approximation to the specific gravity of H. amarus eggs at salinities between 0.5 and 4 ppt. However, the specific gravity of artificial eggs was greater than that for H. amarus eggs at salinities near zero and was substantially less than the specific gravity of H. amarus eggs at 8 ppt salinity.