Citigroup was the top bookrunning underwriter of U.S. municipal bonds in the first half of 2011, according to Thomson Reuters data on Friday.

Citi, which ranked fourth among underwriters in the first quarter of 2011, handled 165 deals totaling nearly $15.3 billion to take the top spot, followed by J.P. Morgan Securities , which was the lead underwriter on 159 deals, totaling $14.3 billion.

Illinois remained the top issuer of muni debt so far this year after it sold $3.7 billion of taxable pension bonds in February, the data showed. The New York City Transitional Finance Authority was the second biggest issuer with nearly $2.7 billion of debt sold in six deals.

Total issuance of $114.7 billion in the first six months of the year marked a 43.8 percent slump from the same period in 2010. However, second-quarter issuance of $68 billion was up 18.6 percent over the first-quarter total of $46.7 billion, Thomson Reuters reported.

(Reporting by Karen Pierog, editing by Gary Crosse)