This paper focuses on the ethical justifiability of patents on Genetically Modified (GM) crops. I argue that there are three distinguishing features of GM crops that make it unethical to grant patents on GM crops, even if we assume that the patent system is in general justified. The first half of the paper critiques David Resnik's recent arguments in favor of patents on GM crops. Resnik argues that we should take a consequentialist approach to the issue, and that the best way to do so is to apply the Precautionary Principle, and that the Precautionary Principle, in this case, supports patents on GM crops. I argue that his argument in favor of a consequentialist treatment is invalid; his Precautionary Principle in any case appears to be incompatible with consequentialism; and his conception of reasonable precautions is too ill-defined to have any argumentative purchase. In the second half of the paper, I argue against GM crop patents, on three grounds. First, there is insufficient evidence to say whether allowing patents on GM crops will make research go faster than not having patents, whilst there is a good reason to think that, other things being equal, a society that allows patents on GM crops will be less just than one that does not. Second, even assuming that patents on GM crops will increase the pace of GM crop research, there is no social need to do so. Third, patents on GM crops will frequently have ethically unacceptable side effects.