This protocol describes microinjection of morpholino oligos (MOs) and RNAs into sea squirt (Ciona) embryos. This is the method of choice for gene disruption assays. MOs that target the initiating ATG can be used, in addition to those that target splice donor and acceptor sites. The latter method permits the selective inhibition of zygotic mRNAs in cases in which the gene in question is expressed in both the egg and embryo. Although injection is usually performed at the one-cell stage, it is possible to target individual blastomeres, up to the eight-cell stage, thereby permitting lineage-specific knockdown of pleiotropic genes. Injection can also be performed in unfertilized eggs to inhibit maternal genes. Microinjection also permits DiI labeling and lineage tracing.