As the sciences of biotechnology, synthetic biology, and nanotechnology develop, questions about liability for harms caused by self-replicating inventions will arise increasingly often. Although negligence, nuisance, and other torts may be relevant in such circumstances, trespass may be the more appropriate cause of action. First, the author explores doctrinal hurdles facing plaintiffs alleging biotrespass. To overcome concerns about the metaphysicality of molecular biotrespass, the author draws analogies to "cybertrespass." To confront the problem of suing patent licensors for the actions of their licensees, the author makes reference to principles of landlord-tenant law. Well-established laws concerning wandering animals shed light on the policies underlying biotrespass. In the second part, the author examines such policies in further detail and considers the purpose of transporting property metaphors into bio/nanospace. The article concludes that biotrespass can be a viable cause of action, if crafted carefully and understood properly. (Contains 2 notes.)