diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
index 07102bac1..7a9ede21d 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
@@ -13,33 +13,33 @@ import kotlin.reflect.KProperty
 
 @Refine
 @Interpretable("FlattenDefault")
-public fun <T> DataFrame<T>.flatten(keepParentNameForColumns: Boolean = false, separator: String = "."): DataFrame<T> =
+public fun <T> DataFrame<T>.flatten(keepParentNameForColumns: Boolean = false, separator: String = "_"): DataFrame<T> =
     flatten(keepParentNameForColumns, separator) { all() }
 
 @Refine
 @Interpretable("Flatten0")
 public fun <T, C> DataFrame<T>.flatten(
     keepParentNameForColumns: Boolean = false,
-    separator: String = ".",
+    separator: String = "_",
     columns: ColumnsSelector<T, C>,
 ): DataFrame<T> = flattenImpl(columns, keepParentNameForColumns, separator)
 
 public fun <T> DataFrame<T>.flatten(
     vararg columns: String,
     keepParentNameForColumns: Boolean = false,
-    separator: String = ".",
+    separator: String = "_",
 ): DataFrame<T> = flatten(keepParentNameForColumns, separator) { columns.toColumnSet() }
 
 public fun <T, C> DataFrame<T>.flatten(
     vararg columns: ColumnReference<C>,
     keepParentNameForColumns: Boolean = false,
-    separator: String = ".",
+    separator: String = "_",
 ): DataFrame<T> = flatten(keepParentNameForColumns, separator) { columns.toColumnSet() }
 
 public fun <T, C> DataFrame<T>.flatten(
     vararg columns: KProperty<C>,
     keepParentNameForColumns: Boolean = false,
-    separator: String = ".",
+    separator: String = "_",
 ): DataFrame<T> = flatten(keepParentNameForColumns, separator) { columns.toColumnSet() }
 
 // endregion
diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/flatten.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/flatten.kt
index 25f8f4c41..bd692ef2f 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/flatten.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/flatten.kt
@@ -15,7 +15,7 @@ import org.jetbrains.kotlinx.dataframe.impl.columns.toColumnSet
 internal fun <T, C> DataFrame<T>.flattenImpl(
     columns: ColumnsSelector<T, C>,
     keepParentNameForColumns: Boolean = false,
-    separator: String = ".",
+    separator: String = "_",
 ): DataFrame<T> {
     val rootColumns = getColumnsWithPaths {
         columns.toColumnSet().filter { it.isColumnGroup() }.simplify()
diff --git a/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
index 6dcd4a93d..aa457c95b 100644
--- a/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
+++ b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/flatten.kt
@@ -98,7 +98,7 @@ class FlattenTests {
 
         aggregate
             .flatten(keepParentNameForColumns = true)
-            .columnNames() shouldBe listOf("city", "mean.age", "mean.weight", "std.age", "std.weight")
+            .columnNames() shouldBe listOf("city", "mean_age", "mean_weight", "std_age", "std_weight")
 
         aggregate
             .flatten(keepParentNameForColumns = true, separator = "_happy_separator_")