What is the ATX file format?
An .atx file is a binary attribute index created by ArcGIS (Esri) for a shapefile or dBASE .dbf attribute table. It is a sidecar file that must reside in the same folder as the .shp and .dbf files it indexes, sharing the same base name with the indexed field name appended - for example, roads_NAME.atx indexes the NAME field of the roads shapefile. One .atx file is produced per indexed attribute field.
The index stores a B-tree or similar structure over the field's values, allowing ArcGIS to accelerate attribute queries and spatial selections without scanning the full .dbf table. ArcCatalog and ArcGIS Pro create and maintain .atx files automatically when the user builds an attribute index; they are not intended to be opened or edited directly.
The format is proprietary and undocumented by Esri. It is incompatible with the older ArcView 3.x attribute index format. If an .atx file is deleted or becomes corrupt, ArcGIS rebuilds it automatically the next time an attribute index is requested - the shapefile data in .shp and .dbf is unaffected.
For modern file geodatabase (.gdb) workflows, attribute indexes are stored internally within the geodatabase rather than as separate .atx sidecar files, making .atx relevant specifically to shapefile-based workflows.
Security & safety
RISK: LOWATX is a binary index file generated by Esri software; it contains no executable code. Safe to handle in a GIS context. If received unexpectedly outside a GIS project context, it has no utility without the associated shapefile.
Format details
in a nutshellPrograms that open ATX files
Windows2 apps
ArcGIS Pro
Paid ATX files are managed automatically - open the shapefile in ArcGIS Pro; the attribute index is used transparently.
ArcGIS Desktop (ArcMap / ArcCatalog)
Paid Open the associated shapefile in ArcMap or ArcCatalog; ATX is used automatically for attribute queries. 
Linux1 app
ArcGIS Pro (via Linux)
Paid ArcGIS Pro supports Linux; shapefile ATX indexes are handled automatically. Technical details
deep spec| File type | Proprietary binary attribute index (B-tree or similar structure) |
| Byte order | Little-endian (consistent with Esri shapefile sidecar conventions) |
| Purpose | Accelerates attribute queries and selections on shapefile .dbf tables |
| Sidecar naming | Shapefile base name + indexed field name + .atx (e.g., roads_NAME.atx) |
| One file per | Each indexed attribute field gets its own .atx file |
| Required co-location | Must reside in the same folder as the .shp and .dbf files it indexes |
| Magic bytes | No publicly documented signature; unverified reports of AAPL header bytes |
| ArcGIS version compatibility | ArcGIS 8+ format; incompatible with ArcView 3.x attribute indexes |
| Rebuild behavior | Automatically rebuilt by ArcGIS if deleted; shapefile data (.shp/.dbf) is unaffected |
| Typical size | A few KB to several MB depending on shapefile record count and field values |
| Format documentation | Proprietary and undocumented by Esri; no public specification |
| Modern alternative | File geodatabase (.gdb) stores attribute indexes internally, replacing .atx for geodatabase workflows |
| Released | 1999 (ArcGIS 8 / ArcCatalog) |
| Latest version | ArcGIS 8 index format (no public versioning) |
| Specification | desktop.arcgis.com |
ATX conversions
Community Q&A
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