The late filing penalty for Form 5304-SIMPLE depends on the size of the employer. - For employers with 100 or more employees, the penalty is $25 per day for each late participant notification, up to a maximum of $15,000 per year. - For employers with fewer than 100 employees, the penalty is $50 per day for each late participant notification, up to a maximum of $7,500 per year. It's important to note that these penalties may change over time, so it's always recommended to refer to the most recent IRS guidelines or consult with a tax professional for accurate and up-to-date information.