- Ice Cream & Frozen Yogurt, Winthrop
- Ice Sports Clubs & Instruction, Winthrop
- Industrial & Commercial Real Estate, Winthrop
- Industrial & Heavy Equipment Repair & Service, Winthrop
- Industrial Goods & Products - Hard Goods - Large, Winthrop
- Industrial Pump Repair & Service, Winthrop - Centrifugal, Positive Displacement, Submersible, Diaphragm, Hydraulic, Water, Vacuum Pumps, etc
- Industry, Winthrop
- Infant Furniture & Equipment Rental, Winthrop
- Information Bureaus & Services, Winthrop
- Insurance Adjusters, Winthrop
- Insurance Adjusters Public, Winthrop
- Insurance Advisors, Winthrop
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- Insurance Brokers, Winthrop
- Insurance Carriers, Winthrop
- Insurance Claim Processing Services, Winthrop
- Insurance Consulting & Information Services, Winthrop
- Insurance General Liability, Winthrop
- Insurance Motorcycles, Winthrop
- Insurance Renters, Winthrop
- Insurance Services, Winthrop
- Internet Services, Winthrop
- Investment Services & Advisors, Winthrop
- Italian Restaurants, Winthrop
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