| | St. Ignatius, MT | Arlee, MT | United States |
| Population | 888 | 725 | 329,725,481 |
| Female Population | 51.4% | 56.6% | 50.5% |
| Male Population | 48.7% | 43.5% | 49.5% |
| Median Age | 33.1 | 36.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 800 | 649 | 309,327,143 |
| Population - 2000 | 788 | 602 | 285,036,114 |
| Population - 1990 | 778 | 489 | 251,960,433 |
| Pop. 2000 to Now | 6.9% | 10.6% | 15.6% |
| Pop. 1990 to Now | 8.2% | 36.2% | 30.8% |
| space4 | | | |
| Population Density | 1,666.0 | 112.1 | 93.4 |
| Land Area | 0.5 | 6.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | St. Ignatius, MT | Arlee, MT | United States |
| Age 0 to 4 | 8.1% | 5.4% | 5.9% |
| Age 5 to 9 | 9.6% | 6.8% | 6.1% |
| Age 10 to 14 | 8.1% | 9.5% | 6.6% |
| Age 15 to 17 | 5.3% | 4.7% | 3.9% |
| Age 18 to 20 | 4.7% | 0.4% | 4.0% |
| Age 21 to 24 | 5.9% | 8.3% | 5.2% |
| Age 25 to 34 | 10.6% | 7.9% | 13.8% |
| Age 35 to 44 | 12.1% | 26.6% | 12.9% |
| Age 45 to 54 | 8.7% | 7.6% | 12.6% |
| Age 55 to 59 | 4.7% | 5.0% | 6.7% |
| Age 60 to 64 | 5.5% | 5.9% | 6.3% |
| Age 65 to 74 | 7.8% | 7.7% | 9.6% |
| Age 75 to 84 | 4.6% | 3.6% | 4.5% |
| Age 85 and over | 4.4% | 0.7% | 1.9% |
| space1 | | | |
| Race | | | |
| | St. Ignatius, MT | Arlee, MT | United States |
| White | 51.7% | 39.4% | 59.4% |
| Black | 0.0% | 0.0% | 12.2% |
| Asian | 0.0% | 0.0% | 5.6% |
| Native American | 33.6% | 55.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.5% | 0.0% | 0.2% |
| Other | 0.2% | 0.0% | 0.4% |
| Two or More Races | 10.4% | 1.9% | 3.2% |
| Hispanic | 3.7% | 3.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | St. Ignatius, MT | Arlee, MT | United States |
| Married Population | 36.3% | 44.9% | 49.8% |
| Currently Married | 32.9% | 42.1% | 48.0% |
| Married but Separated | 3.3% | 2.8% | 1.8% |
| Single Population | 63.7% | 55.1% | 50.2% |
| Never Married | 41.1% | 27.1% | 33.8% |
| Divorced | 10.8% | 14.6% | 10.8% |
| Widowed | 11.8% | 13.4% | 5.6% |
| space5 | | | |
| Household Size | 2.93 | 2.74 | 2.60 |
| Households | 293 | 265 | 124,010,992 |
| Family Households | 182 | 186 | 80,755,759 |
| Married couple, w/children | 29.1% | 40.9% | 30.5% |
| Married couple, no children | 28.0% | 24.7% | 42.9% |
| Non Family Households | 111 | 79 | 43,255,233 |
| Single householder, w/children | 26.4% | 18.3% | 15.8% |
| Single householder, no children | 16.5% | 16.1% | 10.8% |