Technical Abstract: Cyanobacteria grow in California rice fields where they form large mats that may smoother seedlings or cause them to dislodge, resulting in yield loss. The most troublesome species is Nostoc spongiaeforme. It is very difficult to control using currently accepted methods, i.e., aerial applications of copper sulfate. An algicide, Hydrothol 191, has been suggested for controlling nuisance algal growths in California rice fields. The purpose of the experiments described here was to evaluate the effect of Hydrothol 191 on growth of Nostoc spongiaeforme and the green alga, Hydrodictyon sp. In laboratory experiments, Hydrothol 191 reduced Nostoc spongiaeforme growth rates at of 0.3 ppm (parts per million). This effect was removed when rice straw was added to the growth medium. Indicating the rice straw may have introduced bacteria capable of degrading Hydrothol 191. In outdoor experiments, which used rice field water containing decomposing rice straw, and Hydrothol 191 concentrations between 0 and 5 ppm there was little effect on Nostoc spongiaeforme. In contrast, Hydrodictyon sp. exhibited injury symptoms at 1 ppm or greater. However Hydrodictyon sp. recovered by the end of the seven day exposure. It is not clear how this algicide will be useful in the management of Nostoc spongiaeforme or Hydrodictyon sp. in California rice fields.