N-Ethyl-N-nitrosourea (ENU) treatment is the standard experimental method used for chemical mutagenesis of many vertebrate organisms commonly used in the laboratory. In zebrafish and medaka, the use of a repetitive, sublethal dose of 3-3.5 mM ENU has been shown to give the best balance between mutagenicity and toxicity. However, even at this concentration, a significant proportion of fish die during the treatment. Therefore, large numbers of fish are required to obtain a sufficient number of mutagenized founders at the end of the procedure. Additionally, it is quite common to have high levels of mortality in any particular dosing cycle. This may cause a mutagenesis experiment to suddenly fail after several weeks of work. Here we provide a very simple method for ENU mutagenesis of zebrafish using a subparalytic dose of clove oil as a sedative that drastically reduces the lethality of ENU treatment in fish. This facilitates ENU mutagenesis protocols considerably, facilitates higher dosing, and allows for sensitive strains of fish such as homozygous mutants to be mutagenized for use in genetic studies.