The localization of oskar (osk) RNA to the posterior pole of the developing fruit fly (Drosophila) oocyte induces the assembly of pole plasm, causing development of the abdomen and germ line. Failure to localize oskar RNA results in embryos that lack abdomen and germ cells. Conversely, mis-targeting of oskar RNA to the anterior of the oocyte causes formation of ectopic abdomen and germ cells at the anterior pole. Maternal mutants that have reduced pole plasm activity produce sterile adults with normal abdominal development, suggesting that germ cells are more sensitive than abdomen to defects in pole plasm assembly. Thus mutations in genes that reduce oskar RNA localization or activity can be recovered as viable sterile adults. In a screen for mutants defective in germ cell formation, we isolated nine alleles of the tropomyosin II gene. Here we show that mutations in tropomyosin II (TmII) virtually abolish oskar RNA localization to the posterior pole, suggesting an involvement of the actin network in oskar RNA localization.