- Cable TV Companies & Services, Port Jefferson
- Cafe Restaurants, Port Jefferson
- Call Centers, Port Jefferson
- Candy, Nut & Confectionery Wholesale & Retail, Port Jefferson - Candy Stores
- Caning Furniture, Port Jefferson
- Cardiologists, Port Jefferson - Heart Doctors, Physicians & Surgeons (Cardiology)
- Carpenters, Port Jefferson - Carpentry Contractors
- Carpet, Rug & Linoleum Flooring Dealers, Port Jefferson
- Casual Dining Restaurants, Port Jefferson
- Caterers Food Services, Port Jefferson
- Central American Restaurants, Port Jefferson
- Chambers Of Commerce, Port Jefferson
- Charitable & Non-Profit Organizations, Port Jefferson
- Cheese Products, Port Jefferson - Cheese Shops
- Chemical Manufacturers, Port Jefferson
- Chemical Product Manufacturers, Port Jefferson
- Child Care & Day Care Services, Port Jefferson
- Child Related Social Services, Port Jefferson
- Chinese Restaurants, Port Jefferson
- Chiropractor, Port Jefferson
- Christian Churches, Port Jefferson
- Churches 1 Of 3, Port Jefferson
- Churches 2 Of 3, Port Jefferson
- Cigar, Cigarette & Tobacco Store & Supplies, Port Jefferson
- City & County Administrative Agencies, Port Jefferson
- City & County Government, Port Jefferson
- Cleaning & Maintenance Services, Port Jefferson
- Clinics, Port Jefferson
- Clothing Stores, Port Jefferson - Clothes & Accessories
- CNC Machine Shop, Port Jefferson
- Coffee, Port Jefferson
- Coffee & Tea Stores, Port Jefferson
- Coffee, Espresso & Tea House Restaurants, Port Jefferson
- Coin Dealers & Supplies , Port Jefferson
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- Collection Agency Services, Port Jefferson
- Comic Book Stores, Port Jefferson
- Commercial Equipment, Port Jefferson
- Commercial Printing, Port Jefferson
- Commercial Printing Manufacturers, Port Jefferson
- Communications Services, Port Jefferson
- Community & Recreational Facilities, Port Jefferson
- Community Services, Port Jefferson
- Computer & Audio Visual Services, Port Jefferson
- Computer & Laptop Repair, Port Jefferson - PC Repair Services
- Computer & Software Stores, Port Jefferson
- Computer Repair, Port Jefferson
- Computer Software & Services, Port Jefferson
- Computer Software Service, Port Jefferson
- Computer Stores, Port Jefferson
- Computer System Consultants, Port Jefferson - IT Consulting Services
- Computer System Designers & Consultants, Port Jefferson
- Condiments & Sauces, Port Jefferson
- Construction, Port Jefferson
- Construction & Civil Engineer, Port Jefferson
- Construction Companies, Port Jefferson
- Construction Services, Port Jefferson
- Consultants & Services, Port Jefferson
- Control Systems & Regulators, Port Jefferson
- Convenience Stores, Port Jefferson
- Cooking Oils & Shortenings, Port Jefferson
- Costume Sales & Rental, Port Jefferson
- Country Clubs, Port Jefferson
- Court Reporter , Port Jefferson - Stenographer & Convention Reporters
- Credit Unions, Port Jefferson
- Criminal Attorneys & Lawyers, Port Jefferson
- Criminal Defense Attorneys & Lawyers, Port Jefferson
- Criminal Defense Lawyers & Attorneys, Port Jefferson
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