NEW YORK (AP) — Prepaid debit card issuer Green Dot Corp. said Thursday that second-quarter net income fell 17 percent as higher costs offset higher revenue.

The company lowered its guidance for the year, in part because of uncertainty about growing competition. Shares fell 20 percent in aftermarket trading.

Green Dot, which which went public in July 2010, offers prepaid MasterCard and Visa cards that can be reloaded and used for shopping online, paying bills, buying things in stores and withdrawing cash at ATMs.

Prepaid cards are a fast-growing segment of the payments industry. While the cards are aimed mainly at people who don't have bank accounts or whose credit ratings are damaged, they also are becoming popular with consumers seeking to avoid surprise bank fees and keep from accumulating credit card balances.

But the category is growing quickly, and competitors are proliferating. The company cut its guidance in part because it expects that retailers will introduce other prepaid cards later in the year.

Net income for the April-to-June quarter fell to $10 million after paying dividends on preferred stock from $12 million last year. On a per-share basis, earnings were flat at 27 cents because shares outstanding shrank about 16 percent.

Excluding one-time items, net income totaled 35 cents per share. Analysts expected net income of 38 cents per share, according to FactSet.

Revenue rose 19 percent to $136.7 million from $115 million last year. Analysts expected revenue of $141.7 million.

Operating costs rose 23 percent to $117.8 million. Sales, marketing, compensation and benefits, processing and other general costs all rose.

For the year, Green Dot expects profit of $1.29 to $1.32 per share, excluding one-time items. Previously, it forecast net income of $1.65 and $1.70. Analysts expect net income of on revenue of $1.69 per share.

Shares dropped $4.63 to $18.69 in aftermarket trading. The stock closed down 11 cents to $23.32 in regular trading.