In ancient Rome, adsidui (sg. adsiduus; also assiduus, assidui, Latin for "diligent, loyal", and collectively, "taxpayers") were the citizens who were liable to military service in the main line of battle, that is, for much of the history of the Roman republic, as legionaries. The adsidui were the members of the first five census classes, which were, according to the Roman historian Livy, created under the reign of Servius Tullius, the sixth legendary king of ancient Rome. Under Tullius' original organisation, the first class was made of the richest, and thus best-equipped citizens, with helmet, shield, greaves, cuirass, spear and sword. As one went down through the classes and the corresponding levels of wealth, equipment went lighter and lighter. According to Peter Connolly, the goal of Tullius' reform was to base military service on wealth, and not race, thus better integrating the Etruscans, who at that time ruled Rome, and the Romans themselves ; he points out, however, that in the beginning most members of the richest first class must have been Etruscans.
The adsidui were, as opposed to the proletarii, eligible to serve in the legions. However, along the structural evolution of the Roman army, the census levels necessary to be an adsiduus were gradually lowered. In 107 BC, Gaius Marius enrolled the landless capite censi in his army. These Marian reforms are generally taken as the last step in the evolution of the army, allowing any Roman citizen to become a legionnaire,  though some have argued this was a one-off.
- Adsiduus in Charlton T. Lewis, An Elementary Latin Dictionary
- Livy, History of Rome, Book I, chapter 42 Archived October 2, 2009, at the Wayback Machine and Book I, chapter 43
- Connolly, Peter, Greece and Rome at War, pp95-96.
- Connolly, Peter, Greece and Rome at War, p95.
- Connolly, Peter, Greece and Rome at War, p214.
- Rich, The supposed manpower shortage