Casal2 (pronounced Casal-two) has replaced CASAL, New Zealand’s integrated fish stock assessment pack-age since 2002. Casal2 replicates most of CASAL’s functions such as many stocks, fisheries, and areas; age-or length-based dynamics; the use of several data sources (e.g., abundance indices, age frequencies, andtagging), but it has been extended into a generalised modelling tool and it uses more efficient program-ming practices and tools than CASAL. CASAL has enabled a more efficient use of resources for stockassessments, such as allowing a wider range of staff to conduct assessments, helped communicationbetween members of the Assessment Working Groups, enabling a focus on fits of the models and modelstructure since standardised methods meant that model components were known and rigorous testingensured confidence that the code base was validated. Design decisions taken at an early stage in CASAL’sdevelopment now limit its expansion. Re-coding CASAL was New Zealand’s best option to retain the ben-efits of CASAL and to control its own destiny over progress in new stock assessment techniques, such aseasy use in management strategy evaluations. We have replicated many CASAL assessments successfullyand so will release the first version of Casal2 in 2016 as an open source package under the GPL-2.0 licence. A pre-release download of an executable,draft manual, examples, and source code are available from ftp://ftp.niwa.co.nz/Casal2/windows/Casal2.zip.