- Tattooing, Holtsville
- Telecommunications, Holtsville
- Telephone Answering Service, Holtsville - Messenger Services
- Telephone Communication Service & Phone Companies, Holtsville - Local & Long Distance Telephone Service
- Telephone Equipment & Phone Systems, Holtsville - Sales, Service & Repair
- Telephone Equipment & Systems - Repair & Service, Holtsville - Telecommunication Services
- Terrazzo Contractors, Holtsville
- Terrazzo Tile Marble & Mosaic Contractors, Holtsville
- Testing Laboratories, Holtsville - Lab Tests
- Tile Contractors - Install & Repair, Holtsville - Ceramic & Porcelain Tile Flooring - Kitchen & Bathroom
- Tile Supplies, Holtsville - Tile Stores - Companies
- Timberland & Timberland Companies, Holtsville
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- Tires - Wholesale & Retail, Holtsville
- Title Companies, Holtsville - Title Search, Service & Abstract Company
- Tourist Attractions, Holtsville
- Towing Service, Holtsville
- Trailer Rental & Leasing, Holtsville
- Transportation, Holtsville
- Transportation Equipment Manufacturers, Holtsville
- Transportation Rental & Leasing, Holtsville
- Travel & Tourism, Holtsville
- Tree & Shrub Transplanting & Removal, Holtsville
- Truck Rental & Leasing, Holtsville - Rent a Truck & Tractor
- Trucking - Garbage, Refuse, Waste, Trash, Scrap & Junk Removal, Holtsville - Collection, Removal, Disposal, Recycling, etc
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