Original 24 top-level NAICS categories:

Code  Title
11  Agriculture, Forestry, Fishing and Hunting
21  Mining
22  Utilities
23  Construction
31  Manufacturing
32  Manufacturing
33  Manufacturing
42  Wholesale Trade
44  Retail Trade
45  Retail Trade
48  Transportation and Warehousing
49  Transportation and Warehousing
51  Information
52  Finance and Insurance
53  Real Estate and Rental and Leasing
54  Professional, Scientific, and Technical Services
55  Management of Companies and Enterprises
56  Administrative and Support and Waste Management and Remediation Services
61  Educational Services
62  Health Care and Social Assistance
71  Arts, Entertainment, and Recreation
72  Accommodation and Food Services
81  Other Services (except Public Administration)
92  Public Administration

Have been merged into 9 prime NAICS categories:

Code  Title
1  Agriculture, Forestry, Fishing and Hunting
2  Mining, Utilities and Construction
3  Manufacturing
4  Wholesale Trade, Retail Trade, Transportation and Warehousing
5  Information, Finance and Insurance, Professional, etc.
6  Educational Services, Health Care and Social Assistance
7  Arts, Entertainment, and Recreation, Accommodation and Food Services
8  Other Services (except Public Administration)
9  Public Administration