Abstract: The active object programming model is particularly adapted to easily program distributed objects: it separates objects into several activities, each manipulated by a single thread, preventing data races. However, this programming model has its limitations in terms of expressiveness -- risk of deadlocks -- and of efficiency on multicore machines. To overcome these limitations, this paper presents an extension of the active object model, called multi-active objects, that allows each activity to be multithreaded. The new model is implemented as a Java library and relies on method annotations to decide which requests can be run in parallel. It provides implicit parallelism, sparing the programmer from low-level concurrency mechanisms. We define the operational semantics of the multi-active objects and study the basic properties of this model. The benefits of our proposal are highlighted using two different applications: the NAS Parallel Benchmarks and a peer-to-peer overlay. This report presents the multi-active object programming model, its implementation in Java, its formal semantics and properties, and some experiments showing its efficiency.