United States Census Territorial
Territorial censuses were taken to petition for statehood, apportion the legistature, or for taxation purposes. The federal government wanted to know the population of territories to determine if there were enough citizens to apply for statehood. These were generally taken in the years between the federal censuses. These censuses are often incomplete and most are not indexed.
A special census was made in 1885 for Colorado, Florida, Nebraska, New Mexico, North Dakota, and South Dakota. The Family History Library has copies of most territorial censuses. These are listed in the Place Search of the FamilySearch Catalog under "[STATE] - CENSUS - [YEAR]"
State, Territorial, and Colonial Censuses
You can find further information about special territorial censuses in Wiki pages available for each state or territory.
Territorial censuses are important supplements to state and federal censuses. The more censuses used, the more complete a picture of the family a researcher can build. Territorial censuses also can help track individuals in the areas before statehood when other records are scarce.