Antisense oligonucleotides are commonly employed to study the roles of genes in development. Although morpholino phosphorodiamidate oligonucleotides (morpholinos) are widely used to block translation or splicing of target gene products' the usefulness of other modifications in mediating RNase-H independent inhibition of gene activity in embryos has not been investigated. In this study, we investigated the extent that fully modified 2′-O-methyl oligonucleotides (2′-OMe oligos) that can function as translation inhibiting reagents in vivo, using Xenopus and zebrafish embryos. We find that oligos against Xenopus β-catenin, wnt11, and bmp4 and against zebrafish chordin (chd), which can efficiently and specifically generate embryonic loss-of-function phenotypes comparable with morpholino injection and other methods. These results show that fully modified 2′-OMe oligos can function as RNase-H independent antisense reagents in vertebrate embryos and can thus serve as an alternative modification to morpholinos in some cases. genesis 49:117–123, 2011. © 2010 Wiley-Liss, Inc.