Trading volume on Polymarket has declined for the first time in eight months, a setback that comes as its founder acknowledges missteps and its chief rival Kalshi Inc. continues to grow.

Monthly notional trading volume on Polymarket’s offshore exchange and US app slipped by roughly 9% to $10.3 billion in April, according to user-compiled data on Dune Analytics. Kalshi’s volume rose 13% to $14.8 billion.