Speakers in a Warlpiri community in northern Australia are participants in a complex multilingual situation in which there has been a dramatic change in the last thirty years. Children, and adults under approximately age 30, now speak a new bilingual mixed language as the language of their everyday communication. The new language, Light Warlpiri, systematically combines elements from the variety of Warlpiri spoken in Lajamanu (Lajamanu Warlpiri) and Aboriginal English or Kriol (an English-lexified creole). Both Lajamanu Warlpiri and Light Warlpiri are learned and spoken in the community. In both languages grammatical relations are indicated by an ergative-absolutive casemarking system on overt agents and a nominative-accusative system of bound pronouns, and both show variable word order. But in Light Warlpiri ergative case-marking is optional, and word order and pragmatic factors also contribute information about indicating agents. The study shows that there has been intergenerational change in the use of ergative case-marking in Warlpiri, with younger speakers using it on agents less often than older speakers. Both children and adults use ergative marking more often on agents that are postverbal, and children produce this pattern more frequently than adults do, which suggests that they are regularizing a pattern found in adult speech.