Flying solo? Prepare to pay up.

Janet Rahn, a software test analyst from Oakland, California, was shocked when she recently asked for a single rate on an Overseas Adventure Travel tour of Kenya and Tanzania. "As part of the package, they offered a layover in Amsterdam for about $125, based on double occupancy," she remembers. "As a single, I'm being charged $270."

The dreaded single supplement - charging solo guests up to twice a couple's rate - isn't new. If you travel by yourself, and stay in a room or cabin meant for two, you'd expect to pay a little more.

But lately, it seems, travel companies have come to see singles as suckers. For example, Rahn says that in addition to shelling out twice the room rate, Overseas Adventure Travel wanted her to pay a $20 penalty. Indeed, singles are often singled out for punitive fees and surcharges, ranging from extra taxes and port charges to "supplements" that far exceed what two people sharing a cabin or room would have to pay.

"It doesn't make sense," says Rahn. "If I were traveling with someone else, we would be charged a total of $250 for the room. Traveling by myself, I'm charged $270?"

She's right, of course. It doesn't make any sense.

Or does it? I asked Overseas Adventure Travel about the price discrepancy. It turns out the single rate of $270 is correct. But a review of Rahn's reservation records suggests she may have been quoted the wrong price when she phoned the company. "My sales colleague might have given her the estimate of about $120 because she considered Ms. Rahn to be part of a double," spokeswoman Priscilla O'Reilly told me. To its credit, the company agreed to honor its first price quote.

It might be a mistake to write off Rahn's experience as a simple misunderstanding. Vicki Fuller, a retired teacher from New York, says she's been paying single supplements for years, and has noticed a pattern. "They're often 150 to 200 percent over the regular price for double occupancy," she says.

"It feels like discrimination," says reader Melanie Austin, summing up the sentiments of many solo travelers I've heard from.

It often is discrimination. Here are four ways the travel industry is biased against singles -- and what you can do about it.

Your money -- or your vacation

Travel companies sometimes treat their solo travelers worse than cargo. Shannon Kovack and her roommate prepaid for a tour of southern India last year. But just before their trip, her roommate broke her wrist and had to stay home. "I was contacted by the tour company and advised that now that I was a single traveler, I would have to pay an additional $2,990," she says. "I pointed out that our trip was already paid for. Nope, they said -- you have to pay the extra."

Holding your vacation hostage in that way is wrong on so many levels, I don't even know where to begin. But Kovack could have also bought travel insurance, which would have covered the extra money her tour operator was demanding at the 11th hour.

More fees, please

One of the most common complaints I get from cruise passengers is that their single supplement more than doubled the cost of their cruise. When asked, their cruise line says the extra covers taxes and port fees. Some travel agents have suggested that this is nothing more than a money grab by cruise lines, and that those port fees aren't going to the port, but straight to the cruise line's bottom line. I think they're probably right.