- Ice Cream & Frozen Yogurt, Port Jefferson
- Industrial & Commercial Cleaning Services & Companies, Port Jefferson
- Industrial & Commercial Real Estate, Port Jefferson
- Industrial Contractors, Port Jefferson
- Industrial Engineers & Designers, Port Jefferson - Architectural Designers
- Industrial Goods & Products - Hard Goods - Small, Port Jefferson
- Industrial Goods & Products - Soft - Large & Small, Port Jefferson
- Industry, Port Jefferson
- Inspection & Testing Services, Port Jefferson
- Insurance Carriers, Port Jefferson
- Insurance General Liability, Port Jefferson
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- Insurance Motorcycles, Port Jefferson
- Insurance Renters, Port Jefferson
- Interior Designers, Port Jefferson
- Interior Designers & Decorators, Port Jefferson
- Internal Medicine Doctors, Port Jefferson - Internists Physicians & Surgeons
- Internet Marketing & Advertising Services, Port Jefferson
- Internet Providers, Port Jefferson
- Internet Services, Port Jefferson
- Investment Management Services, Port Jefferson
- Investment Services & Advisors, Port Jefferson
- Italian Restaurants, Port Jefferson
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