This paper describes an authoring language for specifying communicative behavior and interaction of agents in virtual worlds. We focus on the popular three-dimensional (3D) multi-user online world "Second Life"and the emerging "OpenSimulator" project. While tools for designing avatars and in-world objects exist, technology to support content creators in scripting (computer-controlled) agents ("bots") is currently missing. Therefore, we have implemented new client software that controls the verbal and non-verbal behavior of bots based on the Multimodal Presentation Markup Language 3D (MPML3D). This paper compares both platforms and discusses the merits and limitations of each from the perspective of adding agents. Categories and Subject Descriptors H.5.1 [Information Interfaces And Presentation]: Multimedia Information Systems--Artificial, augmented, and virtual realities General Terms Human Factors Keywords agents, authoring language, metaverse, virtual w...