Choosing the right mutagen means selecting the right combination of mutagen efficiency and mutagen specificity. For mutagen efficiency, nothing beats EMS. It is extremely potent, it is easy to use, and its mutational specificity is well documented. If mutations other than G/C-->A/T transitions are desired, mutagens other than EMS must be used. Based on initial observations, ENU appears to be as efficient as EMS. Work with other organisms predicts that ENU will yield a wider variety of transitions and transversions than EMS. If this proves to be true, ENU will become an important mutagen for routine genetic analysis. For investigators wanting large multigene deletions, gamma irradiation, UV irradiation, formaldehyde, and DEO are the mutagens of choice. Gamma irradiation yields the highest frequency of events by far, but may also yield more complex rearrangements. Based on limited information, UV irradiation, formaldehyde, and DEO appear to be effective deletion mutagens. Of the three, UV appears to be the most efficient. For investigators wanting small intragenic deletions, TMP appears most effective. TMP is not very potent, but a large proportion of TMP-induced unc-22 mutations are small deletions. Hopefully this will be true of all genes. For investigators wanting other types of genome rearrangements (e.g., translocations, crossover suppressors), gamma irradiation (or possibly X irradiation) is effective. For transposon insertions, mut-2 (especially strain TR679) provides the highest possible frequency of events. Because mut-2 activates several families of transposons, it yields insertions in genes that are poor targets for Tc1. Manipulating a strain with such high frequencies of spontaneous mutations, however, can be problematical (see above). For Tc1-specific events, mut-6 (strain RW7097) is the best choice. It provides frequencies comparable to those of Bergerac, but its Tc1 copy number is much lower. A reasonable strategy for spontaneous mutagenesis is to use TR679 only if mutants are not obtained in strains with lower levels of activity (e.g., MT3126 or RW7097).