Twenty-one sediments collected in urban waterways of the Pearl River Delta (PRD) in China were screened for acute toxicity using Chironomus dilutus in addition to being examined for potential contributors of sediment toxicity, including 19 organochlorine, five organophosphate, and nine pyrethroid insecticides, 28 polycyclic aromatic hydrocarbons, 27 polychlorinated biphenyls, 15 polybrominated diphenyl ethers, 12 metals, and ammonia. Fifteen of the 21 sediments exhibited acute toxicity to C. dilutus, with 33% of the samples exhibiting 100% mortality. This is one of the first studies in China which directly correlates a broad range of sediment-associated contaminants to ecological effects measured by bioassays. A toxic unit approach showed that pyrethroids contributed most to the observed toxicity; as cypermethrin alone was predicted to cause significant mortality in about half of the sites. Specific toxicity identification evaluation analysis confirmed pyrethroid toxicity. Other contaminants may also be supplemental contributors at a few sites. The current study suggests that pyrethroids are the principal cause of contamination and that future risk assessment and mitigation efforts in this area should focus primarily on pyrethroids but should not disregard other contaminants as potential risk is evident.