On August 29, 2005 Hurricane Katrina struck the Gulf Coast and storm surges breached levees flooding much of New Orleans, Louisiana. One month after the storm, sediment was collected and toxicity was tested using Japanese medaka (Oryzias latipes) embryos. Sediments with the highest contaminant levels showed the highest embryonic mortality and most delayed development. However, no sediment caused an increased mutant frequency. When the most contaminated site was resampled in February, 2006 contaminant levels and toxicity decreased. During toxicity testing, approximately 20% of embryos incubated with sediment from one of these sites died and turned red. A red bacterium was isolated that is Gram-negative, cocco-baccilus, non-motile, and most similar to Hahella chejuensis based on genetic and metabolic tests. This bacterium caused 100% infection at 108 bacterial cells per ml and variable infection at lower doses. This study was the first to examine biological effects of exposure to post-Hurricane Katrina sediments.